Was the item placed into the requirements by authorising the sales order? If not the line has been created as a stock requisition and therefore is not linked to a sales order.
If the line has been placed into the requirements by authorising the sales order, is it possible that the line has been added to a purchase order and then deleted off the purchase order?
If the answer is yes... In order to delete a line off a purchase order, you need to open the purchase order and double click into the line you wish to delete. Click into the 'Extension' tab and select the 'Trash' icon, this needs to be performed twice for items linked to sales order. If this is only performed once the item will remain in the requirements as a stock requisition and will be able to be purchased into stock, with the link to the existing purchase order being removed. The item will only remain linked to the sales order if this line is deleted from the extension twice..