I tried to sign up, but it says the username is already taken.
If you have previously signed up for an MITx account, you already have an edX account and can log in with your existing username and password. If you don’t have an MITx account and received this error, it's possible that someone else has already signed up with that username. Please try a different, more unique username – for example, try adding a random number to the end.