Why does this game prompt me for credit card?

I want to buy the US version of Putty for my Xbox One, and I already have enough money in my account, which I had redeemed earlier. However, when I attempt to buy this game, the marketplace prompts me for valid credit card details.

What is the problem and how do I fix it?

The problem is that you're trying to buy the game from the xbox.com site, and it's a game for the Xbox 360.

For some reason (don't ask me why, I have no clue), whenever you try to buy Xbox 360 games via the xbox.com site, you will be asked to provide a credit card, even if you have enough funds in your account to purchase it; that includes free games (Games for Gold). The Xbox One does not suffer from this oversight.

The only workaround I know of, is to buy the game via your Xbox 360 (if you still have one). Otherwise, you'll have to buy this via the Xbox One or the Windows 10 Xbox app, assuming the game is available for purchase there.