You should need check that $order argument is valid instance of the WC_Order Class:

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 );function wc_cancelled_order_add_customer_email( $recipient, $order ){    // Avoiding errors in backend (mandatory when using $order argument)    if ( ! is_a( $order, 'WC_Order' ) ) return $recipient;    return $recipient .= "," . $order->get_billing_email();}

Code goes in function.php file of your active child theme (or active theme). Tested and works.

You could also use instead in this particular case:

// Avoiding errors in backend (mandatory when using $order argument)if ( ! method_exists( $order, 'get_billing_email' ) ) return $recipient;

Related and similar:

• Sending email to customer on cancelled order in Woocommerce
• Add recipients based on user role to failed and cancelled WooCommerce emails

I have found a very nice approach:

First, extend the woocommerce default emails by adding a filter to woocommerce_email_classes:

add_filter('woocommerce_email_classes', function ($classes) {   $classes['WC_Email_Customer_Order_Failed'] = include __DIR__ . "/wc-emails/class-wc-customer-order-failed.php";   // you can add as many classes as you want here   return $classes;}); 

Then, we create the ./wc-emails/class-wc-customer-order-failed.php file. I recommend starting with one of the original WC_Email classes, located under /wp-content/plugins/woocommerce/emails/class-wc-email-{$email-name}.php. Make sure you rename the class after you do it. As the time of this writing I'm using Woocommerce 5.3.0, and the class-wc-customer-order-failed.php should look like this:

<?phpif (!defined('ABSPATH')) {    exit; // Exit if accessed directly.}if (!class_exists('WC_Email_Customer_Order_Failed', false)) {/** * Cancelled Order Email. * * An email sent to the admin when an order is cancelled. * * @class       WC_Email_Cancelled_Order * @version     2.2.7 * @package     WooCommerce\Classes\Emails * @extends     WC_Email */class WC_Email_Customer_Order_Failed extends WC_Email{    /**     * Constructor.     */    public function __construct()    {        $this->id             = 'custom_failed_order';        // add this to send to customer        $this->customer_email = true;        $this->title          = 'This is a custom email template';        $this->description    = 'You can modify this text to appear in the admin area';        // I'm pointing to a new template which I created in the THEME folder        // to use default email file should be admin-failed-order.php        $this->template_html  = 'emails/customer-failed-order.php';        $this->template_plain = 'emails/plain/customer-failed-order.php';        $this->placeholders   = array(            '{order_date}'   => '',            '{order_number}' => '',        );        // Triggers for this email.        add_action('woocommerce_order_status_pending_to_failed_notification', array($this, 'trigger'), 10, 2);        add_action('woocommerce_order_status_on-hold_to_failed_notification', array($this, 'trigger'), 10, 2);        // Call parent constructor.        parent::__construct();    }    /**     * Get email subject.     *     * @since  3.1.0     * @return string     */    public function get_default_subject()    {        return __('[{site_title}]: Order #{order_number} has failed', 'woocommerce');    }    /**     * Get email heading.     *     * @since  3.1.0     * @return string     */    public function get_default_heading()    {        return __('Order Failed: #{order_number}', 'woocommerce');    }    /**     * Trigger the sending of this email.     *     * @param int            $order_id The order ID.     * @param WC_Order|false $order Order object.     */    public function trigger($order_id, $order = false)    {        $this->setup_locale();        if ($order_id && !is_a($order, 'WC_Order')) {            $order = wc_get_order($order_id);        }        if (is_a($order, 'WC_Order')) {            $this->object                         = $order;            // this is the correct place to get_billing_email() working            $this->recipient                      = $this->object->get_billing_email();            $this->placeholders['{order_date}']   = wc_format_datetime($this->object->get_date_created());            $this->placeholders['{order_number}'] = $this->object->get_order_number();        }        if ($this->is_enabled() && $this->get_recipient()) {            $this->send($this->get_recipient(), $this->get_subject(), $this->get_content(), $this->get_headers(), $this->get_attachments());        }        $this->restore_locale();    }    /**     * Get content html.     *     * @return string     */    public function get_content_html()    {        return wc_get_template_html(            $this->template_html,            array(                'order'              => $this->object,                'email_heading'      => $this->get_heading(),                'additional_content' => $this->get_additional_content(),                'sent_to_admin'      => false,                'plain_text'         => false,                'email'              => $this,            )        );    }    /**     * Get content plain.     *     * @return string     */    public function get_content_plain()    {        return wc_get_template_html(            $this->template_plain,            array(                'order'              => $this->object,                'email_heading'      => $this->get_heading(),                'additional_content' => $this->get_additional_content(),                // remove sending to admin, there is another email that does that                'sent_to_admin'      => false,                'plain_text'         => true,                'email'              => $this,            )        );    }    /**     * Default content to show below main email content.     *     * @since 3.7.0     * @return string     */    public function get_default_additional_content()    {        return __('Hopefully they’ll be back. Read more about <a href="https://docs.woocommerce.com/document/managing-orders/">troubleshooting failed payments</a>.', 'woocommerce');    }    /**     * Initialise settings form fields.     */    public function init_form_fields()    {        /* translators: %s: list of placeholders */        $placeholder_text  = sprintf(__('Available placeholders: %s', 'woocommerce'), '<code>' . esc_html(implode('</code>, <code>', array_keys($this->placeholders))) . '</code>');        $this->form_fields = array(            'enabled'            => array(                'title'   => __('Enable/Disable', 'woocommerce'),                'type'    => 'checkbox',                'label'   => __('Enable this email notification', 'woocommerce'),                'default' => 'yes',            ),            // no need to define recipients in the admin panel            // 'recipient'          => array(            //     'title'       => __('Recipient(s)', 'woocommerce'),            //     'type'        => 'text',            //     /* translators: %s: WP admin email */            //     'description' => sprintf(__('Enter recipients (comma separated) for this email. Defaults to %s.', 'woocommerce'), '<code>' . esc_attr(get_option('admin_email')) . '</code>'),            //     'placeholder' => '',            //     'default'     => '',            //     'desc_tip'    => true,            // ),            'subject'            => array(                'title'       => __('Subject', 'woocommerce'),                'type'        => 'text',                'desc_tip'    => true,                'description' => $placeholder_text,                'placeholder' => $this->get_default_subject(),                'default'     => '',            ),            'heading'            => array(                'title'       => __('Email heading', 'woocommerce'),                'type'        => 'text',                'desc_tip'    => true,                'description' => $placeholder_text,                'placeholder' => $this->get_default_heading(),                'default'     => '',            ),            'additional_content' => array(                'title'       => __('Additional content', 'woocommerce'),                'description' => __('Text to appear below the main email content.', 'woocommerce') . ' ' . $placeholder_text,                'css'         => 'width:400px; height: 75px;',                'placeholder' => __('N/A', 'woocommerce'),                'type'        => 'textarea',                'default'     => $this->get_default_additional_content(),                'desc_tip'    => true,            ),            'email_type'         => array(                'title'       => __('Email type', 'woocommerce'),                'type'        => 'select',                'description' => __('Choose which format of email to send.', 'woocommerce'),                'default'     => 'html',                'class'       => 'email_type wc-enhanced-select',                'options'     => $this->get_email_type_options(),                'desc_tip'    => true,            ),        );    }}}return new WC_Email_Customer_Order_Failed();

Please note

In occasion of Woocommerce update, you should check if the customised files have been updated, and its up to you to keep compatibility :)