Please explain what happens and what message you get when you try to pair it. If it is still asking for their Apple ID then they have not cleared the Watch properly.
Then check you have no other sports watches like Fitbit that are paired to your iPhone
Then force restart the watch. i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.
Then Force Restart the paired iphone
Force restart iPhone – Apple Support (UK)
If you are getting a not connected to interbet message, in the iphone, Settings, General, Transfer or reset iphone, Reset, Reset network settings.
Make sure you have no VPN or VPN based adblocker
Then reset the router to factory settings which usually involves a paperclip on a little hole.
Try again.
Then I recommend reviewing the steps in the articles below
Click: How to pair your Apple Watch with a new iPhone - Apple Support or
Click: If you can't pair an Apple Watch with your iPhone - Apple Support or
Click: If your Apple Watch isn't connected or paired with your iPhone - Apple Support