I want to update my iOS but it is not working

Your iPhone 7plus can only be updated to 15.7

You need to have both a WiFi connection and a working cell service, you have neither. You can't use a hotspot to update.

You probably also have a beta version installed and that's why it thinks you have the latest update.

Also, plug your phone in and charge it, better yet, leave it plugged in while updating.

How to uninstall iOS or iPadOS beta software - Apple Support

I am not clear why you did not update to iOS 14 released on September 16, 2020 and iOS 15 released on September 20, 2021 so far and want to update now to iOS 16.

There is something amiss here!