Remove the coupon section for orders that did not use any coupon.

add_action( 'woocommerce_email_before_order_table', 'conditionally_remove_coupon_section_from_email', 5, 4 );

function conditionally_remove_coupon_section_from_email( $order, $sent_to_admin, $plain_text, $email ) {

    if ( ! $order instanceof WC_Order ) {

        return;

    }

    // Get applied coupons

    $coupons = $order->get_coupon_codes();

    // If NO coupons were used, remove the coupon section

    if ( empty( $coupons ) ) {

        remove_action(

            'woocommerce_email_before_order_table',

            array( \ACFWF()->Order_Details, 'coupons_list_email_register_hook' ),

            10

        );

        remove_action(

            'woocommerce_email_after_order_table',

            array( \ACFWF()->Order_Details, 'coupons_list_email_unregister_hook' ),

            10

        );

    }

}