Did you provide Apple with the receipt you got from Amazon when you purchased them? Or did the original owner provide you a copy of the receipt when they purchased them?

Because before Apple will remove activation lock they would want to see a copy of the second.

Apple doesn’t give a shit about the receipt that shows you purchased second-hand, they only care about the receipt that matches their records of where the device was originally sold.

To add to this and clarify:

While there are ways to remove activation lock, it is either with the method you posted, or by providing Apple with a copy/scan/etc of the original (i.e., the one that matches Apple’s records) purchase receipt, that must also clearly show the device’s serial number.