While you are designing a Report in the Reporting workspace, the Pie Chart Style has the 'Display mapping' option disabled.
Clarity 16.4.3
The dimension the widget is based on is a String attribute (or any other data type that does not have Display mappings available in Studio [Classic Administration > Studio > Objects > [object] > [attribute]).
Use a calculated IF/ELSE dimension to convert a string (such as a concatenated lookup value) into a number before applying display mappings.