Send WooCommerce cancelled or failed order email to customers

woo-customer-cancelled-failed-email.php

<?php
/*
* add the customer to the recipient field for the WooCommerce Failed and Cancelled Order email
*/
function wc_cancelled_order_add_customer_email( $recipient, $order ){
     return $recipient . ',' . $order->billing_email;
 }
 add_filter( 'woocommerce_email_recipient_cancelled_order', 'wc_cancelled_order_add_customer_email', 10, 2 );
 add_filter( 'woocommerce_email_recipient_failed_order', 'wc_cancelled_order_add_customer_email', 10, 2 );