To add product SKUs to your order emails open, the email-order-details.php file and locate the following code snippet:
'show_sku' => false,
'show_image' => true,
'image_size' => array( 100, 100 ),
'plain_text' => $plain_text,
'sent_to_admin' => $sent_to_admin,
Find 'show_sku' => and set it to true to enable product SKUs, and set it to false to disable product SKUs. It will add the SKU after the product name in this format: Product Name (#Product SKU).