WordPress “add to menu” button not working

Try looking at the following areas to try and debug;

1. First and foremost, flush the cache for the url, either by hard
   reload cmd/ctrl + refresh > Hard reload. Or via any caching plugin
   you may be using.
2. Use the inspector to look at the console, when changing items in the
   backend and loading the page initially. See if there are any errors.
3. Try setting the CONCATENATE_SCRIPTS to true in your wp-config.
   define( 'CONCATENATE_SCRIPTS', true ); // remove this if it doesn't
solve your issue.
4. Check your .htaccess file isn’t corrupted. Copy the file and name it
   htaccess-copy. Then delete all the text inside the ORIGINAL and save. Then go
   to “Settings > Permalinks” and try re-saving your permalinks.

Let me know how you get on. 🙂