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. Try to remember how you first created the account and login using that method (by email, Google, or Microsoft)
2. Once you have logged in, go to the settings page
3. Scroll down to "Linked accounts" & press "Connect" on the account you would like to use for single sign-on.