This error message commonly appears within Adobe Digital Editions (or other ACS compatible eReaders) when attempting to download/fulfill a book.

A common cause for this error is that the book being downloaded is not part of a valid distributor. The book likely still exists in the system under "All Items" within the admin console, but it was removed from the distributor specified in the fulfillment link (GBLink). To resolve this error, the book must be added back into the distributor that is being specified in the fulfillment link.