How do I remove Activation Lock from a used Apple Watch?

You need to go back to the previous owner and have them remove activation lock. They can do this via their device by removing the device from their find my app or https://www.icloud.com/find. There is no other way to get around this, and Apple won’t unlock it unless you have the original purchase document.

Activation Lock prevents lost or stolen Apple devices from being used by others.

The 2nd hand seller may have known the Activation Lock would prevent you from using the watch and ripped you off. Return the watch and get your money back. If this was through a website such as eBay, report the seller.

M1 Mac mini, macOS 15.2;  Watch, series 10 GPS-only, watchOS 11.2; iPhone 16, iOS 18.2; TV 4th Gen, tvOS 18.2; HomePods, audioOS 18.2 and Airport Express w/ Airplay 2