If you are logged in to multiple Google accounts and try to go through the archive / backup/migrate process, it will fail. Google doesn’t give us the token when a User is logged in to multiple accounts. You will have to log out of all Google accounts except one in order to make the OAuth process successful. Please refer to "Sign in to multiple accounts at once" for more details about Google settings. 

If this doesn’t work please revoke email backup and archiving access for your mailbox and try again.