This error receives when you are trying to create user in portal. At that time if same user has already created then portal will give you this error.

As We have uniqueness check for username so if user with similar username is available in any of the subscriber then it will not  allow you to create the user with same username.

For that you need to create required user with slight changes then it will allow to create the user.

For E.g. If user "Sathish" already created and giving you error, then you need to create it with slight changes like Satish01