Purchase notes are added to Processing Order emails of woocommerce by default. To send purchase notes in admin email you have to use below code. This code will go into your active theme’s functions.php file.
add_filter ('woocommerce_email_order_items_args', 'send_purchase_note_to_everyone');
function send_purchase_note_to_everyone( $args ) {
$args['show_purchase_note'] = true;
return $args;
}