iPhone 7 won’t connect to Apple Watch series 6 with out iOS update but it won’t let me update to the right iOS

Series 6 requires iOS 14 or higher.

iPhone 7 can update as far as iOS 15.8.

What error message appears when you try to update the iPhone 7?

A screenshot....if possible.....would be very very helpful because it will display 5-6 things that need to be checked.

If your Apple Watch 6 is updated to WatchOS 10, you will NOT be able to use it on an iPhone, 7, 8 or X as they are not compatible with iOS 17, which is required for ANY watch running WatchOS 10. That would require you to invest in a newer more modern iPhone. What is the WatchOS version on the Apple Watch you have?