The receipt will not help. Apple does not get involved with third party sales, so it will be up to you to contact the seller to get the issue resolved. Hopefully you know the person who sold you the phone, since it is not recommended to buy an Apple device from an unknown person unless you have physical access to the device to verify that it is not locked. In addition to an Activation Lock, there are many other problems highlighted here that you may come across.
The All Too Common SAD Reality of Buying … - Apple Community