The most common reason for a "username already exists" message is that the username you want is already taken. Since usernames must be unique, you can't select one that's already in use — try a different variation.
Your username gives you a level of anonymity on the platform, letting you take part without your real identity being visible to other users. Once you've registered, you can't change it later, so it's worth picking one you're comfortable using long term — see "Can I change my username?" for more on that.