Not on the beta list?
We're currently in beta; if you haven't applied for access you will receive an authentication error - request access here.
Using the same email address but with a different login method?
If you are trying to do a social login using an email address that you usually use with a username & password, or vice versa, you will see this error.
If you would like to change the way you login, contact support.
If you're still not sure why you're seeing this error, contact support.