Unable to update iPhone to iOS 16

An iPhone 7 cannot run anything higher than iOS 15.8.3. If you need to run anything higher, you will need a newer phone.

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 17.7.3 or 18.2

You can update an iPhone 7 to the latest release of iOS 15 – currently iOS 15.8.3.

About iOS 15 Updates - Apple Support

You cannot update an iPhone 7 to iOS 16 or higher. If you are running the latest release of iOS 15, the phone will tell you that iOS is up to date. (Which it is – for that phone.)