I am unable to update the software on my iPhone 7 Plus

Your iPhone 7 Plus no longer receives the latest iOS updates and cannot be updated past iOS 15.8.3. If you are trying to update to that, you can do so in Settings > General > Software Update. Otherwise, you will need to purchase a newer iPhone that can run a newer iOS.

Your tag line suggest you own an iPhone 7 on iOS 15. iOS 15.8.3 is as far as an iPhone 7 can go. If you were hoping for iOS 16, 17 or 18, that is NOT possible. You'd need a newer, more modern phone to go higher than iOS 15.8.3.