i cant buy an original xbox game with pre-paid cards?

In order to use the currency on your account for backwards compatible games, they have to be purchased on a 360 console.

unfortunately at the moment it's not possible to buy 360 titles on the xbox one with your microsoft account balance.

Since the bc games are purchased under the same rules of the old marketplace you either need an xbox 360 to use your account's currency or you need a method of payment linked to your account to make the purchase (like a credit card)

In order to use the currency on your account for backwards compatible games, they have to be purchased on a 360 console.

Yes buy pre-paid cards from a game shop or similar retailer.

There are two types,xbox credits and gold membership cards, be sure you get the right one.

Pre-paid card credits can not be used to buy gold time.

When making a pre order with a card, payment isn't taken at the time only a week (or so) before the games release. With a pre paid card there is no guarantee the money loaded on to the account will still be available when payment is due, so this is likely
a reason they don't allow pre orders with our paid cards.

As other have mentioned, use the pre paid card to buy Xbox pre paid credit to last in to your account to make the pre order.