Initially stating No authorization required using Adobe. When I try to change it so I can transfer downloaded books by my Nook e-reader it I get an error message stating the Adobe user name and password is associated with another computer. What gives?

When I set up Adobe reader on my computer to be able to download books from a library and then transfer to my Nook e-reader, I initially stating No authorization required using Adobe.  When I try to change it so I can transfer downloaded books by my Nook e-reader it I get an error message stating the Adobe user name and password is associated with another computer.  What gives?