This error can occur if you have previously authorised Adobe Digital Editions (ADE) without an Adobe ID, but now you are trying to associate ADE with an Adobe ID.
The solution is simply to deauthorise ADE and then reauthorise it.
First deauthorise ADE by following these instructions.
Even if you manage to deauthorise within the ADE software you should then also follow the Windows/ Mac OS specific instructions to delete the registry key (Windows) or activation.dat (Mac).
Then once that has been completed you can reauthorise using these instructions.
Finally you need to delete the ebook from your ADE bookshelf and redownload it from your eBooks.com account now that you have your ADE authorisation in order.
If you are having difficulty reauthorising ADE please contact our support team. Please quote the error message when doing so.