If you get this error message, it means you have already created your account in another way. This guide will help you connect up your account so you can use single sign-on in the future.
To fix this error message and use single sign-on in the future:
1. Login using your email and password (you may need to reset your password if you can't remember it!)
2. If it looks like you don't have an account when you login with email, or you get the same error message, try logging in with Google or Microsoft (the one you haven't tried yet).
3. One of the 3 options (email, Google & Microsoft) should work for you. Once you have logged in, go to the settings page.
4. Scroll down to "Linked accounts" & press "Connect" on the account you would like to use for single sign-on.
You'll then be able to use single sign-on to access your account :)