In order to login with your Facebook account, the account needs to be registered on your iPhone from the following:
1.Settings —> Scroll down to Facebook
Tap to see whether your Facebook account is registered.
If not, please tap on "Add Account" and enter your Facebook ID and password.
nana also needs to be allowed to use your Facebook account.
On the same screen, you will see a list of apps that uses your Facebook account.
Please make sure that nana is ON.
If the setup above is completed and you still can’t login, please try deleting and re-registering the Facebook account on your iPhone.
2.Open the facebook app and make sure that you are logged in to your facebook account.
Open the setting screen on your phone, and make sure that your Facebook account is registered on your phone.
If it is, please try deleting and re-registering the Facebook account on your phone.