The Scope, in your case the DataSet, need to be the last parameter in the aggregate, i.e. after the IIf
:
=SUM(IIF(Fields!TaskDescription.Value = "Running", 1, 0), "DataSet1")
i.e. =Sum(<Expression>, <Scope>)
The expression counts the occurrences of the value Running in the TaskDescription
column in the DataSet1
DataSet.
Edit after comment
A quick test showing the expression in action. A simple DataSet with your column:
I've just added a textbox to a blank report with the above expression:
Works as expected on the sample data: