What sort of nook: dedicated eReader or tablet?
How are you trying the download?
Have you authorized correctly?
Both ADE (Digital Editions) if you are using it, and the noon?
If you are using ADE on a computer to do the initial download ...
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Unfortunately, it often then gives misleading error messages about what is wrong.
Try completely removing any authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your Adobe ID.