What is the meaning of username already exists?
A user, alias, or group already exists with that username
For example, a deleted user might have been re-created (either by an admin or due to a directory sync application). If you then try to restore the deleted user, you'll see the message.
Why does it say this account already exists?
If you receive an error message (such as, "Google Account already exists") as you're trying to set up your first Google administrator account, it means that the email address is already associated with another Google service (such as an AdWords account).