I had gotten a used Apple Watch, but there is still an activation lock. I don’t have any contact with the owner.

Hello

You are encountering the About Activation Lock on your Apple Watch - Apple Support feature. Activation Lock is designed to prevent anyone else from using an device if it's ever lost or stolen. Deactivation requires knowing the Apple ID and password that set the feature, or by placing a special request to Apple to reset the device. This Apple ID information essentially defines the current ownership of the device.

Read about 'What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else' - "Use these steps to remove your personal information from a device, even if you don't have it anymore. Or make sure that the watch that you're about to buy from someone else is ready for you to use." Click here --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

If you do not know the required information and you are not the owner, get the owner of the account to remove the lock, or return the device for a refund, or recycle the device. The device is otherwise useless to you.