Activation lock

You will need to get their assistance or if you purchased through a 3rd party (e.g., eBay), try to get a refund. There is no known way to bypass activation lock.

Also see: https://support.apple.com/en-us/HT201441

Unless you can contact the previous owner or if they refuse to help you, then this iPhone will be useless to you.

If you can contact them, then provide them with the following article so that they can remove their Apple ID from this phone:

• What to do before selling or giving away your iPhone, iPad, or iPod Touch - Apple Support