My guess is that your calculated fields already have aggregations on them. Therefore, you'll just need to edit the calculated field that uses the parameter and aggregate the measures that are not already aggregated. For example the edited calculation may look similar to the following:
case [Metric Selection 1a]
when"Impressions" then sum([Impressions])
when "Clicks" then sum([Clicks])
when "CTR" then [CTR]
Hope this helps!
Thanks Tracy! Works..!!