Sorry to hear that you are having trouble purchasing the Minecraft gift. To narrow this down, could you please confirm whether you are able to make any other purchase from the Microsoft Store using the same card, or if the problem happens only when trying to gift Minecraft? .
Please also sign in to account.microsoft.com/services and let me know if you see any overdue payments. If possible, tell me whether you have tried a different payment method, such as another card or PayPal, and whether your bank confirms that it is not blocking or declining the transaction. Please do not share the card number or any other financial information.