Have you followed the steps Authorizing your computer and your Kobo eReader with Adobe Digital Editions
After you’ve installed Adobe Digital Editions (ADE) and created an Adobe ID, you’ll need to authorize your computer and your eReader with ADE. You only need to do this once.
Launch ADE on your PC or Mac.
From the Help menu, click Authorize Computer.
A Computer Authorization window will appear.
Enter your Adobe ID and password.
Click Authorize.
Click OK when Adobe finishes authorizing your computer.
Connect your eReader to your computer.
Tap Connect on your eReader.
In ADE, right-click on Kobo eReader under Devices on the left.
Unfortunately I don't have a Kobo and really I don't even use my Nook that much anymore, as I've switched over to Kindle, but.....
IF I remember correctly, sometimes you have to remove authorizations in order to add a new one or a different one.
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