If this error appears when you try to submit your registration, it means that your username includes at least one character that is not allowed in Clickworker usernames. The most common one is probably the @ sign when users try to use their email address as their username (which is not allowed, as you might have figured out by now).
The only allowed characters for clickworker usernames are
- Letters a-Z
- Numbers 0-9
- Special characters -, _ and .
To give you a few examples of good vs. bad user names:
Hal2001
Steven-1985
Butterfly_65
My.Name.Is.Earl
All of the above are acceptable clickworker usernames. They only consist of letters, numbers and allowed special characters. If not already taken, you could pick them as your username.
$Hal$2001$
-+*Butterfly65*+-
My Username is Earl
Those are usernames which you are not allowed to choose, because they either include special characters we do not allow or blank spaces which also does not work.