Activation lock and can't contact the previous owner

That is not a good thing. You will require the previous owner to either provide that information to you or have that person remove the iPhone from their Apple ID. If neither is possible then can you request your money back? You are not going to be able to use your iPhone without that information.

Axel F.

Return the phone or recycle it.

Without the original owner removing the lock, the phone is useless to you.