Permanently unable to buy minecoins

I tried to buy minecoins today on my one child's iPad, I have tried it many different ways, and always get the error, "Sorry, we were unable to complete your purchase. Maybe check your internet connection?"

This iPad is still able to do other things that require internet, including joining a Minecraft Realm.

On a different child's iPad, I WAS able to buy minecoins today (and never saw this error). The failing iPad failed both before and after this successful purchase. Both iPads are the same model (iPad mini 4), running the same versions of iOS and Minecraft.