Blackboard is ending support for course cartridges after June 2019. Course cartridges were a way for instructors to import a package of course materials from a publisher into their Blackboard course and then customize the materials as needed. Every course cartridge import required a download key from the publisher. Each publisher's contract with Blackboard has a unique end date. Some of these dates have already passed while the latest will expire on December 31, 2019. After that date, course cartridges will no longer work to import content into a Blackboard course.
More information is available at this webpage that describes how course cartridges were used by instructors in the past: Import Course Cartridges; in the bulletin about the end of this feature Blackboard Course Cartridges End of Life support bulletin; and at Blackboard's page: Content Distribution Agreement (CDA) End of Life.