You did not give CashbackEarners.co.uk permission to your email address in Facebook
Without your permission to your email address, it is not possible to log in on CashbackEarners.co.uk with your Facebook account. Please remove CashbackEarners.co.uk in your Facebook account settings and log in again.
In Facebook go to: Settings & Privacy - Settings - Apps and websites and click on the button Remove behind CashbackEarners.co.uk.
After these steps, please try again to login in with Facebook on CashbackEarners.co.uk and make sure to give CashbackEarners.co.uk permission to your email address.