A Dynatrace Portal username must be unique throughout the entire Dynatrace Performance Network, regardless of the account. This means that if the username you want to use is assigned to a user in another account, you will receive the "Username Already Exists" message.
To create a new username, try slightly modifying the username you attempted to use. For example, if user acb already exists, try abc1. Another tactic is to use the user's email address, or full name including their middle name, to avoid the duplication.