Have you also authorize your Reader?
how do I authorize my nook?
Check the help of your nook. This should add in the help.
Sep 2016 - I have only recently bought a Nook (Simple Touch Glowlight 4GB) and have had the error, 'User not activated', with 3 of the eBooks I have bought from online retailers. (I live in the UK and we are not allowed to buy directly from Barnes and Noble !!)
I have been receiving an '.acsm' file then downloading the EPUB file using Adobe Digital Editions. At this point I can read the book on screen on my iMac but when I sideload it on to my Nook, the Nook does not allow me to read it - 'User not activated'.
I have then used an independent, free, eBook program called, 'Calibre', to load the first, downloaded EPUB file into Calibre's own 'library'. At this point I go to the Calibre Library folder on the Mac and copy that version directly on to the Nook.
I don't understand what the difference is or why it works - I did wonder if Calibre stripped out the copyright (DRM) stuff from the eBook but I don't believe it can do that.
If you have received an EPUB file from your library, try loading that into Calibre and using the fresh Calibre version on the Nook as above.
I hope this helps.