How do I pair an Apple Watch 10 with iPhone 16 Pro Max running the latest iOS?

I’m trying to pair a used Apple Watch 10 I just got to my 16 pro max and it’s telling me that I need an updated iOS but the phone is updated. I restarted the phone and still can’t do it. How do I get them paired. I tried to do manually as well but same reply. Thanks

[Re-Titled by Moderator]