How to deactivate another’s Apple ID from a Apple Watch

Unless you can get the previous owner to remove the watch from their account, the watch will be totally, utterly useless to you. Apple will NOT help you. We cannot help you. The only person who can help you is the person who locked the watch to begin with.

If you don't know them, can't reach them or they are unwilling to help, try to get your money back. If you can't get your money back, recycle the watch as it's a useless wrist ornament you simply cannot use.

Read this too --> The All Too Common SAD Reality of Buying … - Apple Community

The original owner needs to remove it for you. See the last section at About Activation Lock on your Apple Watch - Apple Support