Why is my Apple TV asking for confirmation from my iPhone?

On my AppleTV, when I try to download a free app, I click "Get". When I click on that, 2 things happen. My Apple TV gives me an option to "Confirm with Apple Device." And simultaneously I get a grey banner on my phone saying "Confirm Purchase. Use your iPhone to confirm this purchase on (Apple TV)." I click on my phone's gray banner. The banner goes away, and my Apple TV momentarily says “Confirm on (my iPhone)”, then goes to “Payment not completed” and I'm sent back to the app purchase screen. If I choose “Confirm with Apple Device” from the AppleTV, it's basically the same thing. Why isn't this working? Also, this worked about 6 months ago. To troubleshoot, I just signed out and back in--everything looks good. I updated my purchase card info and just downloaded a free app to my iPhone.

[Re-Titled by Moderator]