Waht to do if i can't contact the previous owner?

You seemingly got scammed, as is quite common.

If the seller has disappeared as is typical, recycle this device.

Activation Lock is a theft deterrent, and it wouldn't be a deterrent if it were easily bypassed.

And some number of folks posting requests for bypasses here are likely not the device owner, too.

While the following write-up is for iPad and iPhone, it basically also applies to Watch, too:

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

Apple won't give you anything. Activation lock is an anti theft functionality.

You need to be aware of this as a buyer of used devices

you could try this of course..

start an Activation Lock support request

but I'm guessing you wouldn't have the proof

if you cannot contact the previous owner, then recycle the phone