Purchase an app (billing problem)

I have a problem with my credit card. There has no money in it so i cannot use it.

But i bought an itunes gift card 15 euros in order to purchase tagalicious which costs 9 euros. I redeem the gift card and it says balance 10 euros. I try to buy tagalicious and i cant... it says problem with my credit card.... what my credit card has to do with the 10 euros i have and i can use on my itunes gift card?

Thanks it worked.