Yes -- this is almost certainly that your calling user context doesn't have the permissions in place to make the call, for some reason. We do have this call working fine in our test environments, so I would encourage you to report this through D2L's support desk, and note that it's an API calling issue. You can help move it along significantly if you can provide a packet trace (via Fiddler or Wireshark) of a successful API call, and the not-good call, both outgoing request and response back along with your incident.
If, after opening the incident, you can report the INC number back here in a comment, I can try to expedite assistance at our end.