I bought a used Apple Watch and the owner didn’t unpaid it the activation lock I can’t get into it

Here is how the owner can release the lock:

Before you buy, sell, give away, or trade in - Apple Support

For password recovery:

If you forgot your Apple ID password - Apple Support

If you forgot your Apple ID - Apple Support

Apple’s I Forgot Service

How to use account recovery when you can’t reset your Apple ID password - Apple Support

Contact Apple for help with Apple ID account security - Apple Support

If the owner does not know and cannot recover their password and lacks original receipts, you will want to request a refund. Otherwise, recycle it, as it’ll never be useful to you without the owner releasing that lock.

They should be able to turn off Activation Lock remotely. As far as I know Apple doesn’t support you unless you have proof of purchase. At least that was the case when I worked at Apple.

About Activation Lock - Apple Support