If you see an error in the top right, that says "Unable to connect to Google Calendar" when you enter your employee's profile, you probably already connected this employee to the Google Calendar, but (s)he dropped out.
They usually drop out when the App you created in Google Developers Console is not published, so first access your Google Developers Console, and go to the OAuth Consent Screen. In there, make sure the app is published:
After you do this, you'll need to access your database (via phpMyAdmin, or some other database management tool) and access the wp_amelia_providers_to_google_calendar table (please note that wp_ is the default database prefix, but on your end, it could be anything, so look for your_prefix_amelia_providers_to_google_calendar table). Delete all rows from that table, and after that, you should be able to connect your employees to the Google Calendar.
