Can't pair my Apple Watch to my new iPhone.

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

Then;

Setup your Watch to a new iPhone

Set up your Apple Watch - Apple Support

M1 Mac mini, macOS 12.4; Watch, series 5 GPS/cellular, watchOS 8.6; iPad Pro 2nd Gen, iPadOS 15.5; iPhone 12 Pro, iOS 15.5; TV 4th Gen, tvOS 15.5; HomePods, audioOS 15.5 and Airport Express w/ Airplay 2