With the Kobo plugged in, ADE should prompt you to "authorize" the device. Do make sure you have set the date / time correctly on it.
See
https://wiki.mobileread.com/wiki/Kobo_eReader point
How do I fix the "Content is Locked" message for books with Adobe DRM?
Quote:
To authorize the device and computer:
1. Plug in the Kobo
2. Open ADE
3. You will be prompted to authorize your device with your Adobe ID login
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