Here is the answer. There are 2 issues. First check your settings in apps and make sure there is check in the box requiring verification for all purchases, There should only be a check in the box requiring one after 15 min.
Re: your gift card, to the extent that Apple does except the product, for it to work online you need to register it so that it will be associated with a zip code. Th I instructions are on the box. Hang on to the card, in case you need to make a return. Also, Apple may place a large preauthorization on your card if you place an order and charge the card before the item ships. This is normal but a prepaid card is not the best product for this kind of purchase.
Also, Apple treats all folks who do not provide an acceptable form of payment at the point of sale exactly the same way.