There is not stock code against the item to allocate the stock from.
There is not enough stock available to fulfil the quantity in the requirements. - You can part allocate from the product balances, or raise a stock take to fulfil the remaining quantity on the order.
The item is in stock, however due to the pack size you do not have enough 'each' items in stock to create the pack size in the requirements. - You will need do a stock take to add additional stock.