The error say that the ebook are linked with a other Adobe ID as you enter. Has you open/download the eBook before you authoirze Digital Edition?
JUST WAS STRUGLING AN HOUR HOW TO DEAUTHORISE MY MAC AND THEN I FOUND THIS LINK AND IT WORKED
JUST HOLDING ALT WHEN ACCESING LIBRARY FOLDER WILL MAKE ALL FILES VISIBLE (I HAD NO IDEA MY MAC HIDES FILES FROM ME, KINDA STUPID BUT I REALLY COULD NOT FIND THAT ACTIVATION.DAT FILE, EVEN WITH SEARCHING AND IT IS THERE!!!)
Then the eBooks linked with the empty authoirze. Only solved of the problem is a new download of the reseller of the eBook.