mobileStoreMakePurchase not working for iOS

[Andrew at MidWest Coast Media]

I have released my first project with in-app purchases but only to 50% platform success: Android works but iOS does not. I’m using the directions at https://lessons.livecode.com/m/4069/l/186807-how-do-i-implement-in-app-purchases-in-livecode-apple-appstore but am getting generic errors. 

I had been receiving the dreaded “Cannot connect to iTunes Store” error, but did the test user creation dance a couple times to step past that.
When trying in the simulator with a real user account or registered sandbox user I’m getting a mobileStorePurchaseError of “UNKNOWN_ERROR"
When trying on the device (production build from App Store) I’m getting a mobileStorePurchaseError of “An unknown error occurred”

These are all submitted and approved consumables registered in App Store Connect. Is there anyway to track down a more descriptive error message? Same code is working for Google Play. 

Compiled with LiveCode 9.6.2rc3 on macOS 11.0.1 using Xcode 12.1

—Andrew Bell