You’ve arrived here because PDFpenPro 13 was unable to read your Mac App Store receipt. If you know where your older Mac App Store version of PDFpen or PDFpenPro is located on your computer, the first thing to do is to try again. To try again, click on “Enter your old license to upgrade”:

Then click on “Purchased from the Mac App Store”:

Locate your older Mac App Store app and click Open. Only Mac App Store versions of PDFpen or PDFpenPro can be chosen. If you see a grayed out PDFpen or PDFpenPro, it is not a Mac App Store version.