Why won’t my Apple Watch pair to my phone

There could be many reasons. What series watch? FWIW a lot of people have had trouble pairing or updating a series 3 watch. With the limited memory in a series 3 watch, your best option may be to make an appointment at the Genius Bar in an Apple Store near you and let them look at it. If that is not possible, contact online Apple Support. Either way:

Genius Bar Reservation and Apple Support Options - Apple

See the replies to:

Series 3 Apple Watch Setup Fails At Updat… - Apple Community

Apple Watch 3 is stuck on preparing updat… - Apple Community