I get this same error...
You need to add &f=json
(or &f=pjson
) to the request
I think this is by design. The REST directory for the GeoEnrichment Services endpoint is disabled for most people (including myself). Most of the pay services have this feature, including Routing, Traffic, Geocode, etc.
Hope this helps.
-Noah
The REST endpoint for this service is disabled for browsing with html. You can access the results by selecting f=json format. You will need to have an ArcGIS Online Organizational account to create a token for this service.
Generate token example:
Below is some documentation on methods of how to access this service.