API [Content] - "Unauthorized - Cannot authorize with this service"

Problem Overview

When you make an API call, you receive a 401 unauthorized error with the message, "Unauthorized - Cannot authorize with this service".

Root Cause

The admin setting Disable unpublished apps by default may be enabled.

To verify this setting:

1. In the Admin Console's left pane, Apps
2. Click Custom Apps Tab 
3. Click Settings to verify whether Disable unpublished apps by default is enabled. If it is enabled, ensure you've added the application's API key - also known as client ID - in the "Except for:" text box shown in the screenshot below.