Unable to download, required to fill in billing message

Then it is being rejected by your card issuer for some reason. There are a number of reasons the charge or verification might be declined:

• Insufficient balance or account in arrears
• Account suspended for suspected fraud (not necessarily by you)
• Error in card number
• Wrong expiration date
• Wrong security (CVV) code
• Address in your iTunes account does not match exactly the billing address for the card
• The card was not issued in the country of the iTunes store
• You are connected through a VPN provider that does not terminate in the country of the iTunes store you are connected to.

Here is additional information from Apple→If your payment method is declined in the App Store or iTunes Store - Apple Support

You will have to update your billing information, even if it hasn’t changed. It may need the CVV code again, for example, because merchants are not permitted to store it. And also make sure the expiration date and billing address is correct.

You weren’t charged, that was a pending charge that will disappear in a few days. The credit card networks are rather backward; the only way a merchant can determine if a card is valid is to charge something to it; there is no way to just ask the issuer if the card is valid. You will see this any time you register a card with a merchant; they will submit a minimal “hold” for a non-zero amount just to validate the card, and then either cancel it or let it expire without completing the transaction.