what's wrong with my code?

I think you are looking for the query as follows. Always specify your requirement while posting question.

Check below query.

SELECT TOP 1 e.EmployeeID, (SUM(TotalOrderPrice)*0.1) AS reqTotalOrderPrice, COUNT(o.OrderID) as TotalOrders
 FROM Employees e
 INNER JOIN Orders o
 ON o.EMployeeID = e.EmployeeID
 INNER JOIN (SELECT OrderID,SUM((UnitPrice * Quantity) - Discount) AS TotalOrderPrice
 FROM [Order Details]
 GROUP BY OrderID
 )oi
 ON oi.OrderID = o.OrderID
 GROUP BY e.employeeid
 ORDER BY reqTotalOrderPrice DESC,TotalOrders ASC