Are you seeing an activation lock?
If this is a used device, you will need to contact the seller to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless.
Activation Lock - Turn off Find My iPhone
If you are the original owner, take the device, the original purchase receipt, and a photo ID to the nearest Apple Store.
Genius Bar Reservation US
or contact Apple ID Support. They will need to see the receipt also.
Apple ID Support
Apple ID Support (2)
Apple Support by e-mail or chat Select your device, Battery, Power and Charging, then Power adapter not working to get a chat option.
Apple Support using Twitter