The user tried to ship from a warehouse which has no quantity.  In Sales Order, when user enters order qty, if more than one warehouse is available to choose from (for BUY part), a popup screen will let user to choose one warehouse.  We decided to let user to select the warehouse for BUY part in Sales Order, not in PK.  Therefore, when this message appears users can go to Sales Order, find the record, double click "order qty" field, the warehouse selection screen should pop up, and user should be able to change to another warehouse which has qty to ship.