Activation lock

If you don't know who that is, you don't.

If you can't return it for a refund, turn it over to the police. It's probably stolen.

There is no way to bypass the activation lock. It's useless to you.

Normally you would ask the person you bought it from. But, since you say you bought it off of someone on the street, it would be impossible to find that person. You have been scammed and most likely bought a stolen phone. I would suggest turning it over to the police.

This is a common problem when you buy expensive consumer electronics from “some guy in the street”. Now, you have no recourse. You have a useless device and no way to return it or get your money back.