I figured it out. If you are on an Xbox One console, you can only use your giftcard balance to purchase Xbox One games. If you want to purchase an Xbox 360 games, you must purchase the game on an Xbox 360 console and then you can install the games on your
Xbox One. Here is what happened to me:
I got a $25 Xbox giftcard and redeemed it on my Xbox One console
The games I wanted was Crysis for the Xbox 360 but I was not able to purchase the game using my giftcard balance
So I went to the store from my Xbox 360 (from home go to "games" "A-Z" then flick on the letter that the title of the game starts with. In my case I clicked on "C"
Then I went to Crysis and I was able to purchase it using my giftcard
then I went back to my Xbox One console, went to the store, looked up Crysis, and I was able to install it on my Xbox One
i hope this helps