I can’t get my watch to pair with my phone and my old phone says it’s not paired

Try this:

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.

And follow this procedure:

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