bought an apple watch but it is locked

Either the owner didn't know what to do before selling it or you were scammed into buying a lost or stolen Watch. Unless the original owner unlocks the Activation Lock, you cannot use this watch.

Before you sell your  Watch... -

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

M1 Mac mini, macOS 13.4;  Watch, series 7 GPS/cellular, watchOS 9.5.2; iPad Pro 2nd Gen, iPadOS 16.5; iPhone 12 Pro, iOS 16.5; TV 4th Gen, tvOS 16.5; HomePods, audioOS 16.5 and Airport Express w/ Airplay 2