How do I unpaired an activation locked 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.

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock with their password. If the previous owner cannot or will not remove the lock then the watch is useless to you or anyone else. Apple will not help you find the previous owner. Try to get a refund or dispose of the watch properly.

In the future see the 'Before you buy a watch from a third-party' section of:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support