Sorry for the trouble. What we would recommend is going to Schedule > More > Sync, import, export > Sync Full Slate with an external calendar > Add connection > Sync with Google Calendar, then where you see "Select an account or choose Other to connect a new account" choose "Other" and re-establish the authorization between Full Slate and Google Calendar. This should then load your calendars properly so that you can set up the Google Calendar sync.
If that doesn't work, go to Account > Linked Accounts and remove your Google account by clicking the 'trash' icon. Save your changes and try and set up the Google Calendar sync again.
If you continue to experience any trouble, please reach out to support via the 'support' link in your Full Slate account. Thanks!