You did not give LadyCashback.co.uk permission to your email address in Facebook
Without your permission to your email address, it is not possible to log in on LadyCashback.co.uk with your Facebook account. Please remove LadyCashback.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 LadyCashback.co.uk.
After these steps, please try again to login in with Facebook on LadyCashback.co.uk and make sure to give LadyCashback.co.uk permission to your email address.