Unable to update iOS 16 on my iPhone 7 Plus

An iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU.

(258432)

Correct. Your iPhone 7 Plus no longer supports the latest iOS software updates, and cannot be updated past iOS 15.8.3. You will need a newer iPhone to run iOS 16, or ideally the latest iOS 18.

The reason you can’t update to iOS 16 is because your iPhone is not compatible.