If you're using custom post types and you want to be able to make those pages into MemberMouse core pages or use MemberMouse to protect the page you'll need to ensure that your custom post type has page capabilities. If you're unsure if this is the case you'll want to reach out to your developer to ask them about this. Ultimately, in order to protect pages with a custom post type or make a core page, the custom post type needs to be configured such that capability_type equals "page".
This is the check that MemberMouse does in order to determine if the custom post type is something we can work with:
$postTypeObj = get_post_type_object($post->page_type);
if($postTypeObj->capability_type == "page")
// allow to be core page