Apple has two related features, Activation Lock ( Activation Lock for iPhone, iPad, and iPod touch - Apple Support ) and Find My ( iCloud - Find My - Apple ), which are designed to prevent anyone else from using an iPhone, iPad, or iPod touch if it's ever lost or stolen. Deactivation of either requires knowing the Apple ID and password that set the feature, or by placing a special request to Apple to reset the device. This Apple ID information essentially defines the current ownership of the device.
If you do not know the required information, are not the signed-in owner, or do not have Apple accepted proof of purchase, get the owner of the account to remove the lock (the section: "If you no longer have your iPhone or iPad" in: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support ), or return the device for a refund, or recycle the device. The device is otherwise useless to you. If the owner is unknown and has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.
Here are additional resources if you are the account owner, can find the previous device owner, or have an Apple accepted proof or purchase:
● Read this article to learn how to remove Activation Lock either on the device itself or on the web --> How to remove Activation Lock - Apple Support
or this one for disabling Find My if activation lock has not yet engaged --> Turn off Find My on your iPhone or other devices - Apple Support
● Help with a lost Apple ID or password:
• For help with a forgotten Apple ID (this is likely an active email address or one used in the past), click here --> If you forgot your Apple ID email address or phone number - Apple Support
• For help with a forgotten Apple ID password, click here --> If you forgot your Apple ID password - Apple Support
● Reset device if you can prove to Apple's satisfaction you are the owner:
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt* issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the first owner who does this, or a descendant with legal proof of ownership transfer).
*Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support
