apple watch

The person that you bought this watch from needs to follow all the steps in this link;

Prepare the watch so the buyer can use it.

If they can't or won't then get your money back or recycle the watch and put it down to experience.

The original owner must remove the Activation Lock or the Apple Watch will remain useless forever.

If the original owner doesn’t remove the lock, you likely purchased a previously stolen Apple Watch. Get a refund if possible.

About Activation Lock on your Apple Watch - Apple Support