I can’t update my phone

What Model Name of iPhone do you have? Settings > General > About > Model Name?

What iOS Version is currently installed? Settings > General > About > iOS Version

What message appears when you try to run an update?

You cannot install iOS 16 if you have an iPhone 7, if that is what you are really looking for.

Your tag line suggests you have an iPhone 7, with iOS 15. iOS 15.8 is the absolute highest an iPhone 7 is capable of being updated to. What iOS version were you looking to have?