There is a special subforum for the Android version of Reader
although I think that your question is more related with Digital Editions,
You must activate the Nook. What sort of a Nook?
If a standalone eReader that shows up in the left pane of Digital Editions, then use menu/Help/Authorize Device (Library/Authorize Device on the older more reliable v1.7.2) to ADE with the device plugged in.
Otherwise, read the documentation for the Nook or for the eReader app you are running on it.