Did you get an error to contact iTunes Support to complete this transaction? I did, and it’s because of a new Apple Device I got. I sent them a report about 24 hours before, but they didn’t help at all. The error started when I got a new iPad, and after I registered it, restore it, and sync all my apps, I decided to buy a new game. It’s cheap and I have a valid credit card in there and I’ve been using the same credit card for years. I thought maybe it was close to the expiration date of my credit card. So I emailed myself an iTunes Gift Card with my other iTunes Account, my son’s account actually. That didn’t work. However, what work for me is belowI opened iTunes in my MacBook Air, the latest iTunes. Then I go to my account by clicking on my email address on top right of iTunes and go down to the very bottom to reset all warnings for buying and downloading. Go back to the iTunes Store in iTunes then click on the app I have been wanting to buy. I click on it and bought it. Then the new iPad notification pop up suggesting that I turn on apps syncing in the App Store or Automatic Download. So I did just that on my new iPad, and the newly purchased app downloads. At this point, after the app downloaded, I decided to try to purchase another app. So I did, and it worked this time. No more error and you can now go about an purchase apps without getting an error to contact iTunes support to complete this transaction and all you have to do is 3 steps.
Reset all warnings for buying and downloading
Use iTunes on your Mac or PC to buy the app, not free app, because Free app works on my new iPad without a problem.
Go to your new iPad and try to buy another app
This works even on an account with just Gift Card as your balance.