Either you haven't registered the kobo with your AdobeID, or you have registered it with a different ID from the one used when you first accessed the book.
Once accessed, a book is locked to devices registered with the ID in use at the time of first access.
The exact best steps will depend on the details of what you have already registered and what books you already have locked to (one or more) Adobe IDs.
One option is ...
- First check ADE is properly registered and you can read the book there.
- Then, if the kobo is registered with a different ID, unregister it (ctrl-shift-E or cmd-shift-E on Mac to ADE with kobo plugged in).
- Then re-register the kobo (menu/Hlep/Authorize Device on ADE2.x, Library/Authorize Device on older more reliable v1.7.2)
BUT, be aware the kobo will no longer be able to read books that were associated with the ID it was previously registered with.
If that isn't suitable for you post back saying exactly what your situation with IDs and books and we can suggest the best plan.