If the Login to enrol button isn't working, it's usually because your browser no longer recognises that you're signed in.
Try the following steps:
If you're using a browser other than Google Chrome, you should also clear your browser's cookies as well as the cache before signing in again.
Google provides instructions for clearing your cache here:
https://support.google.com/accounts/answer/32050?co=GENIE.Platform%3DDesktop&hl=en
If clearing your browser cache and cookies doesn't resolve the issue, log a ticket via the ChatBot and include the course you're trying to enrol in and the browser you're using.