Activation Lock

See these Apple reads on activation lock?

Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365

Turn off Activation Lock https://support.apple.com/en-us/HT201441

Apple usually won’t unlock a phone without seeing its serial number on your purchase receipt. You may be able to take the receipt and the phone the to the genius bar (make an appointment https://www.apple.com/retail/geniusbar/) of a brick and mortar Apple Store for help in removing the lock.

You can't remove the Activation Lock. If the previous owner can't/won't remove it, the device is worthless.