If you try to use either the Friendly or Native URL from a file through the Web Interface, and see the above error, this is due to your bucket settings. You will not be able to view the URLs for files online from a bucket that is set to allPrivate. To learn about the different bucket settings, please refer to this page.
In order to use the Friendly or Native URLs for files through the Web Interface, you must change your bucket setting to allPublic. You can edit your bucket settings by going to the Buckets page on your account, click on Bucket Settings, and change the setting to allPublic.