How can i buy a game with a prepaid card

Check on https://commerce.microsoft.com whether the amount is correctly displaying in your account.

Even if the cost of the DLC is greater than what is in your account, it should still show the MSA (Microsoft Account, the money redeemed) as a payment option - but greyed out.

Seeing as you have not mentioned this, I feel it may be a possibility the money is not there, in which case the likeliest reason is you redeemed to another account.

If, however, it is there as a greyed out option, it is because the cost of the content is greater than what is in the account and as Onyx says you will need to pick another payment option. It will then ask you if you wish to apply the money in the MSA first.
Do that and it will first empty the MSA and then take the remainder from your alternative.

On the console it will only show the alternate payment method. This is just in case the cost of the DLC goes over what you have in credit. The credit amount will always be taken first on the console.

Thank you so much another question, if i want to buy a prepaid xbox live gold card, can i use the ASIA prepaid card, or i should buy the US xbox gold prepaid card..

Hi, are you asking if you can purchase the 2 games you want with a credit or prepaid card?

By the way, you can you purchase games without a credit or prepaid card. You can buy a xbox gift card at any major store such as Walmart, Best Buy, Gamestop, etc. Then you redeem the code on your xbox and can use it to purchase the game.

https://support.xbox.com/en-US/billing/prepaid-codes/types-of-prepaid-codes

the hearts and stone DLC for the witcher 3