First, please check that you have entered the number correctly. Be sure to enter your library card number without spaces.
If you're receiving this error message in the app, it may be because you already have a Mango account. Try going to the "Log In" screen of the app and signing in with the same email address you entered before, and your password.
In case the problem persists, please reach out to us at email@example.com with the name and location of your library, and we'll be happy to further investigate.