Activation Lock

Activation lock is turned on in the iPhone , to turn off activation lock Apple ID and password is required that is forgotten , use a Mac and go to iforgot.apple.com and reset the Apple ID password used in the iPhone https://support.apple.com/en-us/HT201487

Then follow the article https://support.apple.com/en-us/HT201441

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.