I can't pair my Apple Watch with my new iPhone

You need to unpair from old iPhone first.

Read here => Unpair and erase your Apple Watch - Apple Support

First, you need to unpair the watch from your old iPhone. Here are two ways to unpair your watch if you no longer have the paired iPhone:

• See the 'How to erase your Apple Watch if you don't have your iPhone' section of:

Unpair and erase your Apple Watch - Apple Support

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

Then setup your watch with your new iPhone:

Set up your Apple Watch - Apple Support

or:

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