why my iphone 7 is not updating?

You need a WiFi signal as well as a cell signal, there is no way around it if you want to update wirelessly. If you can't get cellular service to work, then your only other option is to update using a computer.

Click on this link for instructions on how to update your phone using a computer

Update iOS on iPhone - Apple Support

Regarding no service on your iPhone 7 -

Click on the link below for more information.

iPhone 7 Repair Program for "No Service" Issues - Apple Support

Your screenshot shows no service. Resolve that and see what happens. See if this helps: If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

You’ll want to delete iOS 15.7.6, and re-download the current 15.7.7, too.

Settings > General > iPhone Storage