Error while searching for iOS update on iPhone running iOS 15.8.3

Ensure you are connected to an unrestricted Wi-Fi network.

If you have any add-on VPN apps installed, or any add-on security apps, remove all.

Next, force a restart: Force restart iPhone - Apple Support

Then check for iOS 15.8.4 again.

If you are asking about an iPhone 7, you should be able to update to the latest release of iOS 15 (currently 15.8.4).

About iOS 15 Updates - Apple Support

Updating iOS: common issues and solutions - Apple Community

You will not be able to update to iOS 16 or higher.