Since Apple updated to iOS 12, the ‘Log in with the Facebook app’ button doesn't always work.
Life’s too short to battle error messages, so you can avoid the problem by entering your Facebook login details manually instead.
To do this:
Select ‘Log in with Facebook’
Choose ‘Log in with phone or email’
Enter your Facebook login details
If you still have trouble, get in touch on firstname.lastname@example.org and we'll look into it.