Because you are doing an aggregate in th syntax, the non-aggregate field needs to look like an aggregate too.
IF ATTR([Rolling Previous 3 Month]) THEN Sum([Measure Value]) ELSE NULL END
I tried exactly this before and was getting the same error message along with the other eoor message that I mentioed in my previous email.
ATTR is being called with Boolean did you you mean (integer)?
Please post a workbook.
IF MAX( IF [Rolling Previous 3 Month] THEN 1 END) == 1 THEN Sum([Measure Value]) END
That resolves the aggregate mixing issue and also converts the aggregation of a boolean (which Tableau can be rather picky about) into the aggregation of an integer.
I also removed the ELSE NULL as that is superfluous -- the default if no conditions are met, is NULL.
Hope that helps!
By the way, HAimanti, welcome to the forums!
Thank you so much!