I bought Apple Watch series 5 and I have I phone 6 S but it’s won’t pair with my phone what should I do?

iPhone 6s should pair

iPhone 6 will not pair

If iPhone 6, Apple has a 14 day refund period, and over Christmas this is extended into the 1st week of January

https://www.apple.com/shop/help/returns_refund

Is your phone running iOS 13? Is it up-to-date?

ASC - Late 2012 Mac mini, macOS 10.15.2; Watch, series 5 GPS/cellular, watchOS 6.1.1; iPad Air 2, ipadOS 13.3; iPhone 8+, iOS 13.3; TV 4th Gen, tvOS 12.4.1;  HomePods, audioOS 13.2.1 and Airport Express w/ Airplay 2

Is the phone a 6 or a 6s as you mention both in your post.