Apple iOS 11 has anti-theft and may affect how you are able to master reset your device. Check the software version using Software versions & updates: Apple iPhone & iPad on iOS 11. Additional software versions in Related Links below.
- On the Home screen, tap Settings.
- Tap your Apple ID at the top of the list.
- Tap on your device from the list at the bottom.
- Tap Find My iPhone.
- Tap the slider next to Find My iPhone to disable.
- Enter you Apple ID password and tap Turn Off.