I can confirm that this solution works.solution found:
1/ open the wp-notif.php file
2/ get the purchase code: update_site_option 'wpnotif_purchasecode', 'THISCODE'
3/ Copy paste the code
4/ Return to WPNotif on wordpress, and you will find again this message on TOP "This is a trial version of WPNotif, please register the plugin using purchase code here to use full version. If you have not purchased WPNotif yet, then please visit this here to buy a genuine licensed copy."
5/ click on HERE Link
6/ choose the type of server (production)
7/ paste the purchase code previously retrieved and register
and that's all
It works for me, if other with multisite are interested u can try
Personally, I have changed the purchase code that appears in the file to a random one following the same pattern in hex code: aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaa and I would recommend anyone else to do the same.
You can easily find a serial number generator using your preferred search engine.
Bottom line, it works like a charm. Thanks, Lorval