I can’t pair my new phone with my Apple Watch

Start here, How to pair your Apple Watch with a new iPhone - Apple Support

Follow the steps above,you can't pair it with your new phone until you remove the activation lock,the post above yours tells you how to do that,you don't need your old phone the activation lock is on Apple's servers.

Did you unpair it from the old phone first,a watch can only be paired to one phone at a time;