added to the theme’s function.php file:
add_action( ‘woocommerce_admin_order_item_headers’, ‘display_sku_column_header’, 10, 1 );
add_action( ‘woocommerce_admin_order_item_values’, ‘display_sku_column_content’, 10, 3 );

function display_sku_column_header( $order ) {
echo ‘<th class=”sku”>SKU</th>’;
}

function display_sku_column_content( $_product, $item, $item_id ) {
if ( $_product && $_product->get_sku() ) {
echo ‘<td class=”sku”>’ . esc_html( $_product->get_sku() ) . ‘</td>’;
} else {
echo ‘<td class=”sku”>N/A</td>’;
}
}

// Add SKU column header
add_action( ‘woocommerce_admin_order_item_headers’, ‘display_sku_column_header’, 10, 1 );
function display_sku_column_header( $order ) {
echo ‘<th class=”sku”>SKU</th>’;
}

// Add SKU column content
add_action( ‘woocommerce_admin_order_item_values’, ‘display_sku_column_content’, 10, 3 );
function display_sku_column_content( $_product, $item, $item_id ) {
// Check if the main product has a SKU
if ( $_product && $_product->get_sku() ) {
echo ‘<td class=”sku”>’ . esc_html( $_product->get_sku() ) . ‘</td>’;
} else {
// Check for add-ons
if ( isset( $item[‘yith_wapo_options’] ) && ! empty( $item[‘yith_wapo_options’] ) ) {
$addon_skus = [];
foreach ( $item[‘yith_wapo_options’] as $addon ) {
if ( ! empty( $addon[‘sku’] ) ) {
$addon_skus[] = esc_html( $addon[‘sku’] );
}
}
if ( ! empty( $addon_skus ) ) {
echo ‘<td class=”sku”>’ . implode( ‘, ‘, $addon_skus ) . ‘</td>’;
} else {
echo ‘<td class=”sku”>N/A</td>’;
}
} else {
echo ‘<td class=”sku”>N/A</td>’;
}
}
}