@andy , @ABraun I have exact problem after running RF classification I got a blank raster image and question mark in frequency. I saw our member mention about projection.
(maybe the solution) Then I try to reprojection but I left everything is default particularly the projection is Geographic Lat/Lon (WG84) [Dont choose UTM / WGS84 (Automatic)]. Then the classifier work well. (I dont know what happend next if we export into QGIS)