When signing in with Facebook, you might have not provided us with your e-mail address. Please verify this by tapping the "Review the info that you provide" link during the sign in process, as illustrated below:
If the toggle for your email address is switched off as shown here...
...the app will not be able to find your account and you will see the following error message:
The solution is to make sure that the toggle is switched ON, and try again.