After selecting an app to purchase, assigning to an UEM environment and specifying the quantity, I then clicked on the Get button like I normally would to complete the transaction. This time, however, I was prompted with the below:

I then went to Settings and couldn’t quite figure out what might be missing under Apps and Books  > My Billing Information. Then, my manager randomly hit the Add button and discovered that the phone number field was blank. 

Upon filling in the missing piece, we were able to complete the purchase. Separately, I received an email to confirm the change was made.

This is truly one of those ‘Go Figure‘ moments.