If you are trying to gift content then you can only do so via a credit card.
If you are buying for yourself then even if you are being asked for credit card details then your balance should still be charged for ti. From http://support.apple.com/kb/HT5582 :
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.
If it's an account that you've just created, and you didn't use the instructions on this page when creating it, then credit card details will need to be entered before the account can be used in the store.