Select fields from sub query , but I face field could not be bounded
Based on your comment to the question of whether multiple rows could come out of your subquery for a given account number, you indicated that was not possible.  If that's true, I don't see why you can't simply LEFT JOIN by account number against a derived result of your subquery.

Not tested, but basically like this:

SELECT 
    ROW_NUMBER() OVER(ORDER BY c.FullName ASC) AS No#,    
    a.AcctBranchName, 
    c.FullName,
    c.BirthDate,
    c.Age,
    a.StartDate as 'Account Start Date',
    c.CustStartDate as 'Customer Start Date',
    c.CustomerNum,
    a.Currency,
    a.AccountNum,
    a.ProductType as 'Product Name',
    a.Balance as 'Balance KHR',
    a.BalanceUSDEquiv as 'Balance USA',
    aa.ClosedDate,

    CASE 
        WHEN (a.BalanceUSDEquiv*3 &gt;= 200000) then '200,000'
        ELSE a.BalanceUSDEquiv*3 end as 'Sum Insure in USD',
    CASE 
        WHEN (a.BalanceUSDEquiv*3 &gt; 200000) then 200000*(0.1/100)/12 
        ELSE a.BalanceUSDEquiv*3*(0.1/100)/12 end as 'Premium to be paid'

FROM         
     v_Account AS a 
LEFT join v_Customer AS c 
    ON c.CustomerID = a.CustomerID AND c.Businessdate = a.Businessdate
LEFT JOIN
(
    SELECT AccountNum, ClosedDate FROM v_Account WHERE   
        Category = 'Deposit' AND ProductCode IN ('6003', '6253') 
    AND Businessdate='2018-12-06' AND ClosedDate is not NULL
) aa on aa.AccountNum = a.AccountNum
WHERE 
        aa.AccountNum = a.AccountNum
    AND a.Category = 'Deposit' 
    AND a.ProductCode IN ('6003', '6253')
    AND c.Businessdate='2018-12-05'