activation lock

You acquired your device second hand from someone other than Apple or an authorized Apple reseller without knowing it was in activation lock

The presence of activation lock means that we can't help you. We are not authorized to remove the activation lock from any device. And, under your circumstances, Apple support will not remove it since you cannot provide proof of ownership that they will consider to be sufficient. You would need and have your purchase receipt for the device from an authorized Apple seller with the device's serial number on it (which you can't have).* Also you indicate that your can't contact the previous owner.

Thus, given the information you have provided, the iPhone is useless to you.

*Rarely, an iDevice is purchased activation locked from an authorized Apple seller.

Remove Activation Lock? Nope.

You appear to have gotten scammed.

Activation Lock is a theft deterrent, and it wouldn’t be a deterrent if it were easy to bypass.

It can be removed only with the Apple ID and password, or with proof of original purchase with the serial number and matching ID, or similar proof presented to Apple. None of which you here have.

Contact the seller when possible—contact is apparently not possible here, and disappearing sellers are common with scams—and—if the seller used one—contact the folks running the forum where the seller offered this device. Or contact the local police, if that’s an option for you.

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