I had the same issue with a watch I hadn't used for a while and needed to pair to my new phone. I followed the steps dogr suggested (restart phone and watch, low power mode on phone) and that worked for me.
ellyjay
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution