the issue is related to the gutenburg editor, and is likely a javascript conflict caused by a plugin or theme. It could be that another function is unsetting the 'decodeEntities' variable, hence the 'cannot read properties of undefined' message, or it could be as simple as an error in a block of js code that's breaking things.
your best bet is to install classic editor, get rid of gutenburg altogether, and never look back... even if that doesn't fix the above issue.
this is the function that's breaking:
https://developer.wordpress.org/bloc...html-entities/
without being able to see the site live, i'm not able to troubleshoot further, but I'm 99% sure there's a javascript conflict somewhere.