Have you activated Digital Editions with an Adobe ID (or a Vendor ID)?
Go to the Help menu and select Authorization Information...
A window will appear to display your authorization information. If your computer is not authorized, you'll need to do that before you can read any DRM-protected books. If you don't have an Adobe ID, you can create one here.
I have the same problem and my computer has a Authorization.
Why is still the error showing up?
I had to erase my computer authorization, then re-authorize.
Afterwards, I could add the new purchase to the library.