How do I resolve 'iPhone is out of date' error when pairing an Apple Watch?

I have an IPhone 16 ProMac, iOS is 18.3.2. My old watch died and husband gave my his ultra watch, as he doesn’t use it. I’m trying to pair and am getting this message:iPhone is out out date, This Apple Watch requires a newer version of IOS.

How can I fix this? This is the latest iOS and latest phone. I’ve restarted both phone and watch, Bluetooth is on and WiFi is too.