If you have created a new Apple Account since last using the watch, you need to release the Activation Lock that your old Apple Account has on the watch. Activation Lock prevents lost and stolen devices from being used by others.nYou need to recover the old Apple Account to do so.
If you can't gain access to the old Apple Account, take the watch and your original receipt showing the watch’s serial number, plus your photo ID to an Apple Store and request that they remove the Activation Lock for you.
M1 Mac mini, macOS 15.3; Watch, series 10 GPS-only, watchOS 11.3; iPhone 16, iOS 18.3; TV 4th Gen, tvOS 18.3; HomePods, audioOS 18.3 and Airport Express w/ Airplay 2