Add Applied Coupon Code in Admin New Order Email Template - WooCommerce

This can be done using a custom function hooked in woocommerce_email_order_details action hook (for example) that will display in admin emails notifications the used coupons in the order:

// The email function hooked that display the textadd_action( 'woocommerce_email_order_details', 'display_applied_coupons', 10, 4 );function display_applied_coupons( $order, $sent_to_admin, $plain_text, $email ) {    // Only for admins and when there at least 1 coupon in the order    if ( ! $sent_to_admin && count($order->get_items('coupon') ) == 0 ) return;    foreach( $order->get_items('coupon') as $coupon ){        $coupon_codes[] = $coupon->get_code();    }    // For one coupon    if( count($coupon_codes) == 1 ){        $coupon_code = reset($coupon_codes);        echo '<p>'.__( 'Coupon Used: ').$coupon_code.'<p>';    }     // For multiple coupons    else {        $coupon_codes = implode( ', ', $coupon_codes);        echo '<p>'.__( 'Coupons Used: ').$coupon_codes.'<p>';    }}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

Tested and works...

This is because $coupon_codes is not an array() define it with $coupon_codes=array();