Apple Watch

Your Watch has an activation lock from your old Apple ID. This prevents lost and stolen watches from being used by others.

You will need to recover the old Apple ID to remove the lock and free the watch to use with your new Apple ID.

If you still have your purchase receipt for the watch to prove that it's yours, an Apple Store may help you recover use of the watch.

M1 Mac mini, macOS 12; Watch, series 5 GPS/cellular, watchOS 8; iPad Pro 2nd Gen, iPadOS 15; iPad Air 2, iPadOS 15; iPhone 12 Pro, iOS 15; TV 4th Gen, tvOS 15; HomePods, audioOS 15 and Airport Express w/ Airplay 2

You can ask apple to remove the activation lock online on this page;

Online Activation Lock Removal Tool.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.