getting error in sql when doing sum of two column

Question

This sql query is not working the error are as below 

"Each GROUP BY expression must contain at least one column that is not an outer reference".

Kindly help me if some know solution of it. 

 

SELECT DISTINCT SUM(IncentiveBreakups.ActivationAmount + UniqueActivationAmount) AS Monthly,

IncentiveBreakups.ActivationAmount AS [Activation],

Case when Agents.JoiningDate >= '2018-10-01' THEN UniqueActivationAmount ELSE 0 END AS UniqueActivation

FROM Incentives

INNER JOIN IncentiveBreakups ON IncentiveBreakups.IncentiveID = Incentives.ID

INNER JOIN IncentiveSheets ON IncentiveSheets.ID = Incentives.IncentiveSheetID

INNER JOIN IncentiveSheetUsers ON IncentiveSheetUsers.IncentiveSheetID = IncentiveSheets.ID

INNER JOIN Achievements ON IncentiveSheetUsers.UserID = Achievements.UserID

INNER JOIN Agents ON Achievements.AgentID=Agents.ID

AND IncentiveBreakups.IncentiveID = Incentives.ID

AND IncentiveBreakups.Activations = (SELECT COUNT(DISTINCT Achievements.AgentID) AS Count FROM Achievements

WHERE CONVERT(DATE, Achievements.CreatedOn) BETWEEN '2018-09-01' AND '2018-09-30'

AND Achievements.UserID = 23

GROUP BY IncentiveBreakups.ActivationAmount,UniqueActivationAmount

HAVING SUM(CONVERT(FLOAT, Achievements.GWP)) > 8000)