To find previous owner

I aassume from your question that you have obtained a phone which turns out to be Activation Locked? In this situation only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it).

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.

just need more clarification on the case have you purchased the unit as used or did you found the phone

or what is the main issue that you want to contact the previous owner