This error indicates that G Suite failed to connect to your mailbox with the specified credentials.
The common reasons are:
- The credentials specified are invalid.
- The account has been locked: when accessing the mailbox from multiple geographical regions, Google's servers may think this is an unauthorized attempt to access the mailbox.
Test mailbox credentials:
- G Suite for Businesses account: https://www.google.com/a/domain.com
- Non-G Suite for Businesses account: http://www.gmail.com
Follow these instructions to unlock the mailbox.