The geoprocessing service you are creating uses an unsupported output data format. The result of your service can only be returned as an image by using a result map service.
You need to enable a result map service from the Parameters page of the Service Editor.
If you don't want to return the output as a map image, you need to construct your task in another way to return a supported type. See Input modes and parameter data types for suggestions on returning nonsupported outputs.