Have you tried:
$productmeta = new WC_Product($id);
$sku = $productmeta->post->sku;
As seen here: Reference product SKU on WooCommerce confirmation page
Have you tried:
$productmeta = new WC_Product($id);
$sku = $productmeta->post->sku;
As seen here: Reference product SKU on WooCommerce confirmation page