Magento 2: Extension settings not displayed under configuration

Question

I have installed a Magento 2 extension. It's not showing the fields under configuration settings in Magento 2.

Here is System.xml file:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../Magento/Config/etc/system_file.xsd">
 <system> 
     <tab id="mkt" translate="label" sortOrder="11">
        <label>Namespace Marketo Integration</label>
     </tab>
     <section id = "grazz_auth" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
     <label>Authentication</label>
         <tab>mkt</tab>
         <resource>Namespace_Mkt::Namespace_mkt</resource>
         <group id="graz_settings" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
            <label>Namespace Authentication</label>
            <field id="grazz_secret_key" translate="label"  sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Namespace Secret Key</label>
              <!--  <backend_model>Namespace\Mkt\Model\Callback</backend_model>  -->
            </field>
            <field id="expiry_date" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                <label>Expire On</label>
                <comment>Expiry date for the subscription</comment>
              <!--  <backend_model>Namespace\Mkt\Model\Callback</backend_model>  -->
            </field>
         </group>
     </section>
     <section id="Namespace_mkt" translate="label" type="text" sortOrder="22" showInDefault="1" showInWebsite="0" showInStore="0">
         <label>Settings</label>
         <class>Namespace_settings</class>
         <tab>mkt</tab>
         <resource>Namespace_Mkt::Namespace_mkt</resource>
         <group id="email" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
            <label>Marketo Rest API Authentication</label>
            <field id="enable_ex" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Extension</label>
                             <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
            <field id="host_key" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>End Key Ponit</label>
              <!--  <backend_model>Namespace\Mkt\Model\Callback</backend_model>  -->
              <depends>
                    <field id="enable_ex">1</field>
              </depends>
            </field>
            <field id="client_id" translate="label" type="password" sortOrder="2" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Client Id</label>
                <depends>
                    <field id="enable_ex">1</field>
              </depends>
            </field>
            <field id="secret_key" translate="label" type="password" sortOrder="3" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Secret Key</label>
                <depends>
                    <field id="enable_ex">1</field>
              </depends>
            </field>
         </group>
         <group id="general" translate="label" type="text" sortOrder="5" showInDefault="1" showInWebsite="0" showInStore="0">
           <label>Customer Sync in Marketo</label>
           <field id="enable_ex" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Extension</label>
                             <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
           <field id="mkt_lead_integration" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Customer Sync on Signup</label>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                            <depends>
                                    <field id="enable_ex">1</field>
                            </depends>
            </field>
             <field id="mkt_lead_sync_on_login" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Customer Sync on Login</label>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                            <depends>
                                    <field id="enable_ex">1</field>
                            </depends>
            </field>
         </group>
         <group id="autofill" translate="label" type="text" sortOrder="5" showInDefault="1" showInWebsite="1" showInStore="1">
           <label>Prefill Configuration</label>
           <field id="enable_ex" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Extension</label>
                             <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
           <field id="mkt_autofill_configuration" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Prefill</label>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                            <depends>
                                    <field id="enable_ex">1</field>
                            </depends>
            </field>
            <field id="mkt_autofill_restrict" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Restrict Prefill</label>
                            <comment>Restrict fields from prefill. Write comma separated field names. Eg. firstname,email,phone etc. </comment>
                            <depends>
                                    <field id="mkt_autofill_configuration">1</field>
                            </depends>
            </field>
         </group>
         <group id="dos_prevention" translate="label" type="text" sortOrder="5" showInDefault="1" showInWebsite="0" showInStore="0">
           <label>DOS Attack Prevention</label>
           <field id="enable_ex" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Extension</label>
                             <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
           <field id="enable_dos" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable DOS Attack</label>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                            <depends>
                                    <field id="enable_ex">1</field>
                            </depends>
            </field>
           <field id="allowed_hits" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Allowed Hits</label>
                <comment>No. of hits allowed before DOS prevention</comment>
              <!--  <backend_model>Namespace\Mkt\Model\Callback</backend_model>  -->
              <depends>
                                    <field id="enable_dos">1</field>
                            </depends>

            </field>
            <field id="time_unit" translate="label comment" sortOrder="6" type="select" showInDefault="1" showInWebsite="1" showInStore="1">
                 <label>Number of Units</label>
                 <!-- source model which we created for drop down options -->
                 <source_model>Namespace\Mkt\Model\Config\Source\Dostimeunit</source_model>
                 <depends>
                                    <field id="enable_dos">1</field>
                            </depends>

            </field>
            <field id="time" translate="label comment" sortOrder="6" type="select" showInDefault="1" showInWebsite="0" showInStore="0">
                 <label>Time Unit</label>
                 <!-- source model which we created for drop down options -->
                 <source_model>Namespace\Mkt\Model\Config\Source\Dostime</source_model>
                 <depends>
                                    <field id="enable_dos">1</field>
                            </depends>

            </field>
            <field id="ban_time_unit" translate="label comment" sortOrder="6" type="select" showInDefault="1" showInWebsite="0" showInStore="0">
                 <label>Ban Elapse Time in Hours</label>
                 <!-- source model which we created for drop down options -->
                 <source_model>Namespace\Mkt\Model\Config\Source\Dostimeunithour</source_model>
                 <depends>
                                    <field id="enable_dos">1</field>
                            </depends>

            </field>
            <!--<field id="ban_time" translate="label comment" sortOrder="6" type="select" showInDefault="1" showInWebsite="1" showInStore="1">
                 <label>Ban Elapse Time</label>
                <source_model>Namespace\Mkt\Model\Config\Source\Dostime</source_model>
            </field>-->
         </group>
         <group id="orderconfig" translate="label" type="text" sortOrder="5" showInDefault="1" showInWebsite="0" showInStore="0">
           <label>Order Configurations</label>
           <field id="enable_ex" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Extension</label>
                             <frontend_model>Namespace\Mkt\Block\Disable</frontend_model>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
           <field id="productinstruction" translate="label"  sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <!-- add a new field with id scope -->
                    <frontend_model>Namespace\Mkt\Block\Productfield</frontend_model>       
            </field>
           <field id="productcustomobject" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <!-- add a new field with id scope -->
                    <label>Product Custom Object Api Name:</label>
                    <backend_model>Namespace\Mkt\Model\Callback</backend_model>

                            <depends>
                                    <field id="mkt_order_integration">1</field>
                            </depends>
            </field>
            <!--<field id="orderinstruction" translate="label"  sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                 <frontend_model>Namespace\Mkt\Block\Orderfield</frontend_model> 

            </field>-->
            <field id="ordercustomobject" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                    <!-- add a new field with id scope -->
                    <label>Order Custom Objecct Api Name:</label>

                            <depends>
                                    <field id="mkt_order_integration">1</field>
                            </depends>
            </field>
             <field id="mkt_order_integration" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                            <!-- add a new field with id scope -->
                            <label>Enable Order Integration</label>
                            <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                            <depends>
                                    <field id="enable_ex">1</field>
                            </depends>
            </field>
         </group>
         <group id="graz_about" translate="label" type="select" sortOrder="9" showInDefault="1" showInWebsite="0" showInStore="0">
            <label>About Namespace</label>
              <!--  <backend_model>Namespace\Mkt\Model\Callback</backend_model>  -->
            <field id="orderinstruction" translate="label"  sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                 <frontend_model>Namespace\Mkt\Block\Orderfield</frontend_model> 

            </field>
         </group>
     </section>

 </system>
</config>