Add a column with coupons used on admin Orders list in Woocommerce

There is some errors and mistakes in your code… Try the following instead:

add_filter( 'manage_edit-shop_order_columns', 'woo_customer_order_coupon_column_for_orders' );function woo_customer_order_coupon_column_for_orders( $columns ) {    $new_columns = array();    foreach ( $columns as $column_key => $column_label ) {        if ( 'order_total' === $column_key ) {            $new_columns['order_coupons'] = __('Coupons', 'woocommerce');        }        $new_columns[$column_key] = $column_label;    }    return $new_columns;}add_action( 'manage_shop_order_posts_custom_column' , 'woo_display_customer_order_coupon_in_column_for_orders' );function woo_display_customer_order_coupon_in_column_for_orders( $column ) {    global $the_order, $post;    if( $column  == 'order_coupons' ) {        if( $coupons = $the_order->get_used_coupons() ) {            echo implode(', ', $coupons) . ' ('.count($coupons).')';        } else {            echo '<small><em>'. __('No coupons') . '</em></small>';        }    }}

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

(This can't be a sortable column)