Why can't I use just my Gift Card balance on Amazon?

I added my 25$ gift card to my Amazon Balance in anticipation of getting a book on my kindle to read while I'm on a road trip Wednesday. I went on to get the book "In His Own Write" By John Lennon, and it said that I had to have a credit card. I went to ask my dad and he let me put it on but I couldn't buy anything with it. I said okay and went ahead and added it. I proceeded with trying to purchase the eBook, and it told me I had to use the card. I know I couldn't do that so I took off the card and told my dad. We don't know why I can't use JUST my Gift Card Balance, can you tell me a way that I would be able to without using my dads card?

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