Hi Atkinsj
welcome to the Apple (user-to-user) Community
Re: Pairing watch with new iPhone 12
Apple support has an article below which offers tips using a couple of different methods:
As it is a new iPhone to the Watch:
How to pair your Apple Watch with a new iPhone - Apple Support
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More info., if needed:
Re: the iPhone,
if you haven't already, you may need to update to the latest iOS 16.5.1
About iOS 16 Updates - Apple Support
The Watch may also need to be updated to the newest Watch OS version possible, in order to connect with the phone..
You may be able to update the Watch using the phone:
If you wish:
you can read the article below carefully to check the compatibility
Eg: Update your Apple Watch - Apple Support
If both iPhone and Watch have the newest possible updates, pairing should no longer be a problem.
For more support, if needed:
Apple Watch - Official Apple Support
and
iPhone - Official Apple Support
All the best :-)