How to remove Activation Lock on iPhone 11 Pro Max with proof of purchase?

The box makes no difference. Acceptable proof of purchase is the receipt provided by Apple or Authorized Apple seller that shows your name and serial number/IMEI number of the device. Apple does not get involved in third party sales and those receipts do not prove ownership.

Is it possible that your friend who set it up placed the device into Lost Mode? They can do that remotely with the Apple ID/Password used when it was set up. That would be the only reason where I could see that the lock would not be able to be removed, unless the device was previously managed by a school or employer, but I would expect that is not the case either.

Sorry, I don't have any other information that would be helpful to you.

• Where was it purchased?
• Does the receipt show your name and the serial number or IMEi number?