Bought used Apple Watch but it has activation lock.

The previous owner will have to disable the Activation Lock:

About Activation Lock on your Apple Watch - Apple Support

Or, if you have proof of purchase, try to contact Apple Support to get help:

Request additional support for unlocking your device - 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.