Okay… Literally, only half an hour after I posted this I located the problem. In case anyone else ever has the same issue, the problem was that I didn’t have “In-App Purchases” enabled in “Signing & Capabilities.” You have to go to Project > Targets > Signing & Capabilities. There is a tiny, light gray + at the top of the window to add capabilities. Search for “In-App Purchases” and double click the item. No other configuration is required.

I don't see that anyone has posted a resolution to this issue. I've had the problem for a week. I have a support ticket from last week that's gone unanswered. It kind of makes it hard to put out good quality product if you can't get your app into the hands of testers.