What should I do if I couldn’t contact to the previous owner ?

Return it and get your money back.

If you can't do that, turn it over to the police.

There is no way to bypass the activation lock.

The phone has an activation lock on it and you need to get the apple id from the person you got if off,if you cant then you need to get a refund as the only person that can unlock that phone for use is the person you got it from.