open file core/lib/cart.php... go to line 247 and add sku into array
$result = array(
'product_id' => $product_query['product_id'],
'name' => $product_query['name'],
'model' => $product_query['model'],
'shipping' => $product_query['shipping'],
'option' => $option_data,
'download' => $download_data,
'quantity' => $quantity,
'minimum' => $product_query['minimum'],
'maximum' => $product_query['maximum'],
'stock' => $stock,
'price' => ($price + $option_price),
'total' => ($price + $option_price) * $quantity,
'tax_class_id' => $product_query['tax_class_id'],
'weight' => $product_query['weight'],
'weight_class' => $product_query['weight_class'],
'length' => $product_query['length'],
'width' => $product_query['width'],
'height' => $product_query['height'],
'length_class' => $product_query['length_class'],
'ship_individually' => $product_query['ship_individually'],
'shipping_price' => $product_query['shipping_price'],
'free_shipping' => $product_query['free_shipping'],
'sku' => $product_query['sku']
);
Then open core/lib/order and add sku there (line 266):
$product_data[] = array(
'product_id' => $product['product_id'],
'name' => $product['name'],
'model' => $product['model'],
'option' => $option_data,
'download' => $product['download'],
'quantity' => $product['quantity'],
'price' => $product['price'],
'sku' => $product['sku'],
'total' => $product['total'],
'tax' => $this->tax->calcTotalTaxAmount($product['total'],$product['tax_class_id']),
'stock' => $product['stock']);
now you can see sku on your template, i mean storefront/view/default_html5/template/pages/checkout/confirm.tpl ...or old default template