In most cases, you will receive this message after you have already successfully applied the code once, but have subsequently encountered a payment isssue.
In this cases, your order has already been created, but now is in a "Payment Pending" state. You should not repeat the checkout sequence, but simply go to your previous order and complete the payment again. To do this:
1) Go into Account
2) Click on Order History and locate the order that you applied the code to. Click on Pay Now.
Then simply go ahead and make your payment again and complete your order. Your order will then be accepted using your original promotion code and begin processing.