You need to contact Adobe Support either by chat or via phone when you have serial number and activation issues.
Here are some links to help make contact:
Moving this discussion to the Adobe Digital Editions forum.
It sounds like your Nook has not been authorized. Which operating system are you using? If you right click on the Nook within Adobe Digital Editions are you given the option to authorize the device?
Try these steps, hope it works for you.
1. Connect Nook device to your computer.
2. Open the device files on your computer.
3. Move the ".adobedigitaleditons" folder to desktop and delete from your device.
4. Safely eject the device and close Digital Editons if you have opened.
5. Launch Digital Editions (ADE) , connect Nook.
6. Re-check if the Nook is Authorized.
7. Try transferring the Ebooks to your Nook and check if you are able to view them on your Nook device.
If you get the "User not Activated" error there is no need to carry out these extensive and time consuming fixes. Just:
1) Plug your Nook into your computer
2) Open Adobe Digital Editions and make sure you can see your Nook above the Bookshelves
3) Select the Settings icon just above it and choose "Activate"
After doing all this I was completely restored - even the current page in the book I had side-loaded and was part way through reading was still correct.