According to the arcpy.mapping.ExportReport docs:
The ExportReport function has a dependency on the ArcMap installation. Therefore, ExportReport can only be executed on machines that have ArcMap installed. ExportReport will not run using stand-alone installations of ArcGIS Engine or ArcGIS Server. Moreover, ExportReportwill not work as a Geoprocessing Service.
Please incorporate this functionality into stand-alone installations of ArcGIS Server so we don't have to install Desktop on our servers.