I would recommend you create a support request in the forum before writing a negative review as I am happy to assist you. You can write a support topic here: https://wordpress.org/support/plugin/wp-custom-admin-interface/
I also want to note that this very issue has been raised more then once in the forum, so you can already see the answer to your request in the forum. One of the advantages of the forum is you can see if other people have asked questions before which saves everyone time.
I also want to note that we don’t obligate people to buy the plugin. Our plugin on WordPress.org is totally free and includes free support via the WordPress.org forum. Thanks
When a plugin is inactive, its pages under /wp-admin/ simply don’t exist.
Unfortunately, the standard WordPress message in this case doesn’t distinguish between “This admin page is currently not defined” vs. “There could be such a page, but it’s not available to you.”
I saw exactly the same message a couple of times today: I activated a plugin, but it immediately disabled itself since it wasn’t able to write to a certain directory in the file system. But on the first pageload after activating, I still had the links to its admin pages visible on my screen, but I got exactly that message “Sorry, you’re not allowed to access this page.”
I’ve suggested an improvement in WordPress core administration for this particular case: https://core.trac.wordpress.org/ticket/57925