Hello Snow,
When we apply drilldown on a pie chart, it passes the dimension selected (here display) as the parameter. This is what is selected in the “Set Category” dropdown in the drilldown modal window.
The Parameter Mapping section allows you to pass additional parameters (other than the category) to the child report. You can pass existing filter values from the parent report using the filter number like Filter_0, Filter_1 etc. You can also pass static values in the parameter mapping section. For example, country=India.
In the example attached above, you are passing static parameter value for the same variable that you are passing as a category (value of the pie selected). Hence you are getting the error.
You do not need to pass anything in the Parameter Mapping.
Thank You,
Helical Insight.