Pair new Apple watch with iPhone 6+

The current shipping Apple Watches come with watchOS 6 which cannot pair with an iPhone 6 Plus. These watches requires an iPhone 6s, SE, 7, 8, X, XS, XR, 11 or 11 Pro.

If you got your Apple Watch after Nov 15, you have until January 8th to return it for a full refund.

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

If you got your Apple Watch before the 15th, then you have 14 days from the date of purchase to return your Apple Watch for a refund.

Yes, but the requirements are on the Watch box. The only way you can purchase one that will work with your iPhone is to buy a used older model that has not been updated to watchOS 6 and don't update it.

Apple sells lots of products that don't work with some older models of PCs, iDevices, Macs, etc. and always have. As Bob suggested, return it and get your money back.