Locked Apple Watch

If you are not the original owner, you have to ask them to perform all of the steps in this article What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

See also this article for reference

How to remove Activation Lock - Apple Support

Unfortunately, if the original owner cannot remove the activation lock you cannot use the Apple Watch

See this link for further reference https://discussions.apple.com/docs/DOC-250001038?commentId=250004101025

The person that you bought/got this watch from needs to follow all the steps on this page;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.