Make sure the FactBox is open when you have the sales order up:

Enter the item you want to sell into the sales order line, then look in the FactBox. You may have to scroll down:

Here is my understanding of item AVAILABILITY in this area:
"Item Availability" is an aggregate of what is actually in the building minus what is on other sales orders plus what is on purchase orders. More on this later.
"Available Inventory" is the actual inventory in the building. It does not include what is on sales orders currently, but will change when you invoice/ship an order.
"Scheduled Receipt" is the amount that you have on an open Purchase order. This number is added to to "Available Inventory" to get "Item Availability" (less any quantity that is on an open sales order).
Our company does not do backorders so this can get confusing sometimes, especially if a vendor is taking a long time to deliver our product. Below is an example of what it looks like when we have an item on order and low actual counts.
