Pair Apple Watch to new iPhone

If it's a used watch, are you sure that you're encountering isn't Activation Lock?

Activation Lock prevents a lost or stolen Watch from being used by someone else.

Only the original owner can remove the Activation Lock from the watch. Otherwise, it's just a paperweight.

M1 Mac mini, macOS 12.1; Watch, series 5 GPS/cellular, watchOS 8.3; iPad Pro 2nd Gen, iPadOS 15.2; iPad Air 2, iPadOS 15.2; iPhone 12 Pro, iOS 15.2; TV 4th Gen, tvOS 15.2; HomePods, audioOS 15.2 and Airport Express w/ Airplay 2

The person you got it from needs to follow all the steps on this page;

What to do before you sell, give away or trade in your Apple Watch

If they can't or won't get your money back as the watch will be of no use to you.