Hello there, it's possible you don't have any payment method (card, PayPal, etc.) on file beyond just the Microsoft balance. Whenever you are making a purchase on the Microsoft store, whether the purchase is free or using your Microsoft balance, it still requires that a payment method be on file. It won't charge the card or other option you have attached, but it does require that it be there.
It is also possible that the payment method was removed or expired within the last few months, so I would recommend starting and double-checking there. Here are careful directions on how to update your payment method for your Microsoft account.
If, after confirming that you have a valid payment method connected, you still can't make the purchase, then you will need to contact Xbox Support for further assistance with your account.
