Typically helpful ADE error message.
Something to try.
Have you registered your AdobeID with ADE (Digital Editions).
If not, do so and try the download again.
If you have; ADE may have got it muddled up.
Try ctrl-shift-D to ADE (cmd-shift-D if on Mac) to remove any existing possibly muddled registration.
Reregister your AdobeID.
Try the download again.