Is this phone carrier-locked? Meaning that it will only let you get cellular service from a particular carrier (in this case, Verizon)? In that case, you must contact that carrier to remove the lock. Only they can remove it, if they choose to do so.
Or is it Activation Locked, displaying a message like "Locked to Owner"? Activation Lock is an anti-theft measure designed to make stolen and lost iPhones useless to thieves, fences, and anyone they might subsequently sell the phones to. If the phone is Activation Locked with an Apple ID and password that you don't know, we can't remove the Activation Lock for you – and Apple won't, without proof of original purchase that you ("I don't know who own this phone before") obviously don't have.
See: The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community