Why can I not pair my Apple Watch series 2 with apple 12

Have you removed the watch from your old phone?.

You can only pair 1 watch to one phone,so if it's paired with your old phone it won't pair with the new one.

About Activation Lock on your Apple Watch - Apple Support

Is it giving you an error? What happens when you try to troubleshoot using these steps?

https://support.apple.com/en-us/HT205025