I purchased a used series 7 watch from a store and it is locked to an owner that I don’t know. What can I do?

There is nothing you can do unless the original owner unlocks that watch. It is likely a stolen watch. Apple will not help you unlock that watch. Return it for a refund if you can. I recommend reading the article below before thinking of buying another used Apple device.

The All Too Common SAD Reality of Buying … - Apple Community

Return it to the store you purchased from for a refund if possible. Nothing else you can do