Allow editor user to full permission to access plugin settings

This is a problem with the original plugin. They obviously never actually tested this filter. They have used the mc4wp_admin_required_capability to control whether the admin page is accessible, but WordPress still prevents you saving settings unless this filter is also used:

https://developer.wordpress.org/reference/hooks/option_page_capability_option_page

You should contact the plugin author and ask them to fix this, but in the meantime you could implement this yourself. Assuming this is the plugin in question, this should work:

add_filter(
    'option_page_capability_mc4wp_settings',
    function( $capability ) {
        return 'edit_pages';
    }
);

You will probably also need this to save the “Integrations” settings.

add_filter(
    'option_page_capability_ mc4wp_integrations_settings',
    function( $capability ) {
        return 'edit_pages';
    }
);

Plugins like “Advanced Access Manager” don’t usually help with this, because the only way to know whether these filters are possible is to read the plugin code, which is why plugins like that are pretty useless. All they can see are which admin menus are registered, and which user roles are registered, because they’re declared up front. They can’t actually control how a plugin uses roles and capabilities without inspecting the code of every plugin and implementing solutions manually.

If it’s possible grant full access to a plugin’s settings with a plugin like
Advanced Access Manager, then the original plugin has not implemented settings in a very secure way.