1. Make sure you are connecting to the Internet.
2. Sign out account and Quit Office apps.
3. Go to the Library folder and open Group Containers. Ctrl+click each of these folders if present, and Move to Trash.
UBF8T346G9.ms
UBF8T346G9.Office
UBF8T346G9.OfficeOsfWebHost
4. Restart Office then sign in with your account.