Why is my Face ID no longer working for App purchase

If you still need help, reset Face ID, then set it up again. Go to Settings > Face ID &Passcode and tap Reset Face ID. Then tap Set up Face ID to set it up again.3 days ago

If Face ID isn't working on your iPhone or iPad Pro - Apple ...

Do you have a disabled Face ID alert on your device?

Please read the "What to do Next" section in this link, if nothing works as per the instruction given in this link --> If an alert says Face ID has been disabled on your iPhone or iPad Pro