How to fix: this._addClass is not a function

I recently ran into this WordPress problem specifically when opening the Slider Revolution plugin, version 6.4.3. The plugin was outdated. Maybe this was fixed in a newer release, but since I did not update it, I followed another workaround.

After searching quite a bit, I found the solution. For more information, see https://wordpress.org/support/topic/javascript-error-uncaught-typeerror-this-_addclass-is-not-a-function/.

To fix it, install and activate the plugin https://br.wordpress.org/plugins/enable-jquery-migrate-helper/. That should solve the problem.

The reason for the error is that WP 5.6 changed the jQuery version in the JavaScript library it uses, and that can break plugins installed on the site.

Update: The problem came back, and simply installing the plugin was not enough. In the admin bar, I had to go to jQuery Migrate > Setting Page and change the jQuery version to Legacy.