1. open file public_html/storefront/model/checkout/order.php and go to line 575
2. replace
"SELECT oo.*, po.element_type, p.sku, p.product_id
with
"SELECT oo.*, po.element_type, COALESCE(po.sku,p.sku) as sku, p.product_id
3. replace model to sky in your view\default\template\mail\order_confirm.tpl
<td class="align_left"><?php echo $product['sku']; ?></td>