Apple Watch

You can’t. The previous owner must unpair the watch and remove Activation Lock by entering their Apple ID password, so you can use the watch. What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support 

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.

Neither the users on this forum or Apple can help you,only the original owner can help you.