If you're having trouble with your Facebook login on an iOS device, it's highly likely that the issue is related to one of two problems:
1. You may need to re-enter your password on Facebook on your iOS device
Whenever you change your Facebook password, all apps associated with your Facebook account cannot access your information until you re-enter the latest password. It's critical that you re-enter your password in order to be able use the Checkout 51 app.
To do this, simply go to Settings on your device then scroll to Facebook. You should be prompted to re-enter your password.
2. If your password isn't the issue, you may have unintentionally disabled the Checkout 51 app
To re-enable the app, go to Settings > Facebook and look for Checkout 51 in the list of applications. If the Checkout 51 app is not set to "on," this will prevent you from being able to log in.