what's wrong with my code?

SELECT TOP 1 e.EmployeeID

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 TotalOrderPrice * 0.1 DESC,COUNT(o.OrderID) ASC

it gives error likes this:

Msg 8127, Level 16, State 1, Line 11

Column "oi.TotalOrderPrice" is invalid in the ORDER BY clause because it is not contained in either an aggregate function or the GROUP BY clause.