How to open a locked Apple Watch

It sounds like you purchased a used device which is “Activation Locked.”

This is an intentional, designed-in, theft deterrent feature of the device which makes it unusable to anyone other than the registered owner.

Until the prior owner performs the steps outlined in the article below, the device will remain a non-functioning “paperweight” to you.

How to remove Activation Lock - Apple Support

Without the password for the original Apple ID, you're going to need a hammer and a chisel.

I'll bet that the seller knew EXACTLY what they were selling you, a lost & found or even a stolen watch. Hopefully, for your sake, they were the original owner but didn't realize what they needed to do before they sold it.

Sell, give away, or protect a lost Apple Watch - Apple Support

Reach out to the seller and see if they can help you. If they can't, or if they never reply, then see if the website will help you (i.e. eBay Money Back Guarantee).

For future reference, read through this discussion:

https://discussions.apple.com/docs/DOC-250001038