Verification Required Loop - Figured it out!

When downloading an app on an old iPhone, it would bring me to a verification page that only showed my billing information. If I clicked continue it would just throw me back into the loop. Instead, if you click the back button, it’ll show you the full verification page, on which billing information is at the bottom. Turns out I needed to add payment information, which was at the top of the page. After filling this in, I was able to download apps.