It could be that the app you are trying to delete is among Apple's default apps. If this is the case, then you can't delete some of them. Default apps such as App Store, Safari, Clock, Photos, and some others cannot be deleted from any iOS device. However, you can delete the likes of Camera, FaceTime, Calculator, and Mail. And download them again in the future from the App Store. But for third-party apps, they can all be deleted.