What is wrong with this code in SQL?

SELECT 
    `Date`, `Client name`, `Paid Amount`, `Received Amount`,   
    @balance := @balance + (`Received Amount` - `Paid Amount`) AS `Balance` 
FROM 
    (SELECT  
         ord.order_date AS `Date`, ord.client_name AS `Client name`, 
         0 AS `Paid Amount`, ord.paid AS `Received Amount` 
     FROM 
         `orders` AS ord 
     JOIN 
         SELECT 
             cus.customer_id, cus.customer_name  
         FROM 
             `customer` AS cus ON ord.client_name = cus.customer_id 
         UNION ALL
         SELECT 
             sub.cash_date AS `Date`, sub.cash_type AS `Client name`, sub.cash_pay AS `Paid Amount`, sub.cash_rec AS `Received Amount` 
         FROM 
             `cash_value`)  AS sub 
JOIN 
    (SELECT @balance := 0) AS balance_init 
ORDER BY 
    `Date` ASC;