apple watch ultra 2 won't pair with iphone 12 pro max

Your iPhone software is the latest version on general release so it should pair and setup without any problems. Take a look at this support document and use it as a guide to what should happen.

Set up and pair your Apple Watch Ultra with iPhone - Apple Support

If the problem continues please post back to explain what you see with your eyes to tell you it is not working. Don't interpret anything, just go with the facts as you see them. Saying it won't work is not much help, saying what happens is much more useful.