When I try to log into the offline reader I get a 'user not found' error?

When I try to log into the offline reader I get a 'user not found' error?

Before you can log into Cambridge Spiral, you must first open a book on the web reader at least once by going to the book’s main page and clicking on the ‘Read Online’ button – this will link your Cambridge Core/Higher Education account with Cambridge Spiral, allowing you to log into the app and add your books for offline reading.