It is because the user uses the same email address registered a personal Google account before. For now the same username becomes a Google Apps account, the user need to remove or rename the old personal account.

conflicting accounts

The next time that user signs in to a consumer product, they'll be asked to change their account. For example, if has a conflicting account with your organization, Jane would see these options when she signs into a consumer product:

  • Change to an account with a Gmail address ( or another available Gmail name). 
  • Change to an account with a different email address (uses her address). 
  • Sign in with a temporary username Google provides (