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 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 .