Check your product by going to Seller Admin>View/Edit/Delete Product, select the product and click Edit. On the first edit screen look to see if you have Inventory Control enabled. If you do then click Next to get to the More Options screen. On the More Options page in the upper right is an option for Limit Product Sales to, this is your Inventory Control, if you do not have a number in here, then our system will read that you do not have any product in stock. And will give your buyers a message that the product is out of stock.
Since you are selling a download, you can also just disable the Inventory Control option.