Apple Watch 7 and iPhone 6s iOS 15.7.2 not pairing

Hello, I have an Apple Watch 7 and an iPhone 6s iOS 15.7.2

When I try to pair I get an error message.

I have tried automatic and get an error "iPhone Out of Date. This apple watch requires a newer version of iOS".

When I try manually. I enter the 6 digit code, it spins for a bit then says unable (there is an error message that pops up too quickly to read).

Help!

All apple literature says these are compatible.

:(