My system.xml in /etc/adminhtml is the reason for this error
<field id="vendor_role" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
<label>User Roles</label><source_model>Admin\Role\Model\System\Config\Source\Roles</source_model>
</field>
Warning: DOMXPath::query(): Invalid expression in /home/mysite/public_html/vendor/magento/framework/Config/Dom.php on line 273