Hello munyafashukanu,
Thanks for this extra information. When you were notified that this item was available for download, did you have enough credit in iTunes to complete the purchase?
Even if you had credit on your account when you pre-ordered, that amount wouldn’t actually be charged until you download the item when it becomes available. Is it possible that you spent this credit on other items prior to this item becoming available?
If you did not have enough credit on your account (or another valid form of payment such as a credit card) at the time of attempting to download this item, this would explain the alert you are receiving.
The following resources will help you check to see what your iTunes Store balance is, and how to update your billing information if necessary.
See your iTunes credit balance
Change or remove your Apple ID payment information
Best,