If you won, but now you don't have any withdrawable credits, it is because you have already used the credits toward other orders.
When you place an order, whether it is a manual order or a Smart Order, we use any credits in your account before charging your payment card. On the checkout screen, it says how much is used in credits and how much is charged to your card on each transaction, as well as in the email confirmation you receive after your order is placed. You can also view your transaction history from the side menu of the app, it shows which orders your credits were applied to.