iPhone unable to update to iOS 15

What model iPHone do you have? If you have an iPhone 6/6+, you can’t update it past iOS 12.5.6. It doesn’t have the hardware for anything later than that.

If you’re already running iOS 12.5.6, your device is not supported with iOS 13 or newer. There are no hardware upgrades. Your choices are a newer (used, refurb) or new iPhone. iPhone 8 and newer are minimally required for iOS 16, and newer still for all of the features of iOS 16.