I have credits - why can't I buy songs?

Due to recent security issues and identify theft (mostly due to phishing scams), the iTunes Store seems to be requiring more frequent verification of credit card information when a card is registered in your account, even if you have a balance from a prepaid card. Either just go through the verification process or, if you have a credit card entered and don't expect to need it anytime soon, go into the Payment information in your account and set the payment method to "none" which should then stop the regular requirements for verification (it may still ask - mine does - but you should just be able to click through the buttons without problems).

Hope this helps.

How do you know it won't let you?

itunes uses the credit card info to verify the account and location. Just because it asks for this info does not mean that it will be billed to the credit card.