I have faced a similar issue and in my case, the issue was because I was incorrect tab id matching in system.xml

<tab id="custom_tab" translate="label" sortOrder="2000">
            <label>Custom Tab</label>
        </tab>
        <section id="custom_section" translate="label" type="text" sortOrder="40" showInDefault="1" showInWebsite="1" showInStore="1">
            <class>separator-top</class>
            <label>Custom Section</label>
            <tab>customtab</tab>

To fix the issue, I changed
<tab>customtab</tab>
to
<tab>custom_tab</tab>

I have faced a similar issue and in my case, the issue was because I have incorrect tab id matching in system.xml

<tab id="custom_tab" translate="label" sortOrder="2000">
            <label>Custom Tab</label>
        </tab>
        <section id="custom_section" translate="label" type="text" sortOrder="40" showInDefault="1" showInWebsite="1" showInStore="1">
            <class>separator-top</class>
            <label>Custom Section</label>
            <tab>customtab</tab>

To fix the issue, I changed
<tab>customtab</tab>
to
<tab>custom_tab</tab>