It means the item's Tracking dimension group is configured to use 'serial number' and that the serial number is not allowed to be blank> It also means that the item's serial number group is not configured to auto-create serial numbers.
A common scenario is that you will enter the serial number given to each item by the vendor, and this will be done during the receiving process (i.e. before attempting to post the product receipt).
Do you know if this item is meant to have Serial enabled? Do you use Purchase line registration, Item arrival journals or something else in your receiving process?