Exception #0 (Exception): Warning: Undefined array key "id"

Question

i have error when i go to stores->configuration after i create my system.xml file

system.xml

  <?xml version="1.0"?>
  <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
<system>
    <section id="sega" translate="label" sortOrder="20" showInDefault="1" 
 showInWebsite="1" showInStore="1">
        <label>sega</label>
        <tab>SEGA</tab>
        <resource>Magento_Backend::system</resource>
        <group id="general" translate="label" type="text" sortOrder="10" 
showInDefault="1" showInWebsite="1"
               showInStore="1">
            <label>General Configuration</label>
            <field id="enable" translate="label" type="select" sortOrder="10" 
   showInDefault="1" showInWebsite="1"
                   showInStore="1">
                <label>Enable Module</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
            <field id="coins_back_percent" translate="label" type="text" sortOrder="20" 
  showInDefault="1"
                   showInWebsite="1"
                   showInStore="1">
                <depends>
                    <field id="enable">1</field>
                </depends>
                <label>Percent of purchase</label>
                <validate>validate-number validate-greater-than-zero</validate>
            </field>
            <field id="enable_pdp_message" translate="label" type="select" 
     sortOrder="30" showInDefault="1"
                   showInWebsite="1"
                   showInStore="1">
                <depends>
                    <field id="enable">1</field>
                </depends>
                <label>Show message on PDP</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
        </group>
    </section>
    <section id="payment">
        <group id="coins" translate="label" sortOrder="100" showInDefault="1" 
  showInWebsite="1" showInStore="1">
            <label>Coins Payment Method</label>
            <field id="active" translate="label" sortOrder="10" type="select" 
  showInDefault="1" showInWebsite="1"
                   showInStore="1">
                <label>Enable</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
            <field id="title" translate="label" type="text" sortOrder="20" 
  showInDefault="1" showInWebsite="1" showInStore="1">
                <label>custom</label>
            </field>
        </group>
    </section>
</system>

Answer 1

Your tab "SEGA" is missing. You must declare a new tab or point your sections to an existing tab when declaring section.

To add tab to your system configuration, add the code below:

<tab id="SEGA" translate="label" sortOrder="300">
    <label>SEGA</label>
</tab>

Finally, your system.xml looks like this:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <tab id="SEGA" translate="label" sortOrder="300">
            <label>SEGA</label>
        </tab>
        <section id="sega" translate="label" sortOrder="20" showInDefault="1" 
     showInWebsite="1" showInStore="1">
            <label>sega</label>
            <tab>SEGA</tab>
            <resource>Magento_Backend::system</resource>
            <group id="general" translate="label" type="text" sortOrder="10" 
    showInDefault="1" showInWebsite="1"
                   showInStore="1">
                <label>General Configuration</label>
                <field id="enable" translate="label" type="select" sortOrder="10" 
       showInDefault="1" showInWebsite="1"
                       showInStore="1">
                    <label>Enable Module</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="coins_back_percent" translate="label" type="text" sortOrder="20" 
      showInDefault="1"
                       showInWebsite="1"
                       showInStore="1">
                    <depends>
                        <field id="enable">1</field>
                    </depends>
                    <label>Percent of purchase</label>
                    <validate>validate-number validate-greater-than-zero</validate>
                </field>
                <field id="enable_pdp_message" translate="label" type="select" 
         sortOrder="30" showInDefault="1"
                       showInWebsite="1"
                       showInStore="1">
                    <depends>
                        <field id="enable">1</field>
                    </depends>
                    <label>Show message on PDP</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
            </group>
        </section>
        <section id="payment">
            <group id="coins" translate="label" sortOrder="100" showInDefault="1" 
      showInWebsite="1" showInStore="1">
                <label>Coins Payment Method</label>
                <field id="active" translate="label" sortOrder="10" type="select" 
      showInDefault="1" showInWebsite="1"
                       showInStore="1">
                    <label>Enable</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="title" translate="label" type="text" sortOrder="20" 
      showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>custom</label>
                </field>
            </group>
        </section>
    </system>
</config>