On Catalina, you need to use the Retroactive app to install iTunes 12.6.
Ironically, for those that don't currently have an iPad, I think it might be easier to buy the app on a Windows computer these days than on a Mac.
If someone has a better way, I'd love to hear it.
I downloaded that app and got iTunes working with App Store. Many thanks for that link. It is a godsend for those looking to get access to iOS/ iPadOS App Store on the Mac to take advantage of sales.