Why is my iPhone unable to pair with my Apple Watch?
Switching to new watch on iphone app failed after pairing . unable to reset and/or remove pairing to start over
[Re-Titled by Moderator]
iPhone 12, iOS 18
Question marked as
Did you try to erase the watch directly, as described in the second part of this article?
Unpair and erase your Apple Watch - Apple Support
Did you make sure that your iPhone is also updated to the latest iOS 18.1.1 version?
Force restart your iPhone as well:
Force restart iPhone - Apple Support