Cannot find either column "dbo" or the user-defined function or aggregate "dbo.fnDiscountPrice", or the name is ambiguous
Assuming that the column name returned from dbo.fnDiscountPrice is DiscountPrice you could use it like this.

CREATE FUNCTION dbo.fnItemTotal 
    (@ItemID INT = 0)
    RETURNS Table

RETURN (SELECT O.ItemID, SUM(DP.DiscountPrice * O.Quantity) AS TimeTotal
        FROM dbo.fnDiscountPrice(@ItemID) as DP
          CROSS APPLY OrderItems as O
        GROUP BY O.ItemID)