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<br/> 
`<tab>customtab</tab>` <br/>to <br/>`<tab>custom_tab</tab>`