It looks like you’ve got a JS conflict which is stopping anything in the admin area that uses it from working. First thing to do, would be to check the theme version you’re on, it should be 5.5. Then disable any third party plugins, then recheck to see if the issue persists, as it could be a js conflict with code being inserted by a plugin.
As a last resort, if the above doesn’t work, and you’re using the latest version of WordPress (some users have reported minor js issues since the latest wp release – the dev team are working on an update) and still experiencing the problem. You can try rolling back to a previous version of WordPress from https://wordpress.org/download/release-archive/
A step by step guide on how to install can be found here – http://tbwhs.com/blog/wordpress-rollback/