Is this a new account ? If it is then I assume that you didn't use these instructions when creating the account : Create or use your Apple ID without a payment method - Apple Support
If you enter valid payment details you should be able to them remove them and download free items - the card won't be charged, except for a temporary store holding charge to check that the details are correct and valid, which the card issuer should automatically remove within a few days or so (the actual time varies by card issuer)
