I want help fix this SQL code

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

SELECT (a.NumUnit*(SELECT  sp.PriceUnitSale
FROM    SalePrice sp
        INNER JOIN
        (
            SELECT  ProNo, MAX(PriceLastDate) max_date
            FROM    SalePrice
            GROUP   BY ProNo
        ) spt ON  sp.ProNo =a.ProNo AND sp.PriceLastDate = spt.max_date))Amount

FROM    StoreBalance a
        INNER JOIN
        (
            SELECT  ProNo, MAX(BalanceDate) max_date,MAX(BalanceNo) max_No
            FROM    StoreBalance
            GROUP   BY ProNo
        ) b ON  a.ProNo =b.ProNo AND
                a.BalanceDate = b.max_date and  a.BalanceNo = b.max_No and year(BalanceDate)='2019'