Stuck in a limbo due to failed App Store payment verification.

IPhone 7 Plus user here. I tried purchasing a Udemy course, and after my guardian gave me permission (I had to “Ask to Buy”), I was taken to a page in the App Store. It asked me to “Verify Payment,” and apparently I had to enter the security code for my guardian’s credit card. But when I pressed “Done,” it didn’t accept. My guardian realized that he had to update his credit card info (his card was relatively new), but even then the verification didn’t work.

I force closed and then reopened App Store, but it was a big mistake. I could not go back to the verification page. When I went back to the Udemy app, I tried re-purchasing the course, but it said “my previous purchase is still processing.” 

Is there a way I can get out of this limbo? At least redo the purchasing process for the Udemy course? Or do I just have to accept my fate and purchase a different course?