This error can be caused by not having API group permissions available on the API admin group. 
  1. Determine which API admin group is connected to the credentials you are using.
  2. Go to Constituent 360 > Groups > Administrator Group List tab.
  3. Search for the group name.
  4. Select Edit permissions under Actions.
  5. Select Group Administration category from the drop down.
  6. Mark the radio button for Use Luminate Online Group APIs.
  7. Save your changes.
Once completed the authorization error will not occur.