Line items cannot be added to approved orders. First un-approve the order and then add a line item.
To add a line item to an order, click the Add Line Item button. Enter the quantity and SKU for the new line item, then click OK. Or click Cancel to leave the order unchanged. The program adjusts the amount due for the order, taking into account the new quantity and SKU, plus any sales taxes, discounts, surcharges, restocking fees, etc.