Visual Editor Not Clickable

Hello,

We can’t add content using the Visual Editor and most of our site users are not familiar with adding appropriate tags with just the text editor. When I try to click, there’s an error which says:

jquery.js?ver=1.12.4:2 Uncaught Error: no such method ‘instance’ for autocomplete widget instance

Here are the links for the images of the errors:
https://snag.gy/R7KVTi.jpg
https://snag.gy/nJrX2f.jpg

I can see that it’s definitely in conflict with the JS scripts of WP tinymce. We have tried the following:

– disabled all plugins
– disabled the plugins one by one
– reinstalling WordPress
– reinstalling just the WP TinyMCE files

All of those did not work.
FYI, this might be related our theme getting out of date because it was customized and no support for update. I’ve read on some articles that the latest WP Version uses a different JS now(correct me if I’m wrong) and that both of them should use the same JS, so the incompatibility between WP and the theme might be the problem.

I hope someone could help.

Thanks!
-Eli