Why do I get a warning message that my username is invalid?

Every user must select a username for their account. The username can only contain letters, numbers, underscores, and hyphens. We recommend replacing all [spaces] with either an underscore or hyphen.

Incorrect: MY Username
Correct: MY-Username
Correct: MY_Username

Or, it says that the username selected is already in use.

Usernames are unique. Two users cannot share the same username. If you get a notice during registration that your username is already in use, you will need to select a different username.