How are you getting the book on the device? From the Kobo app or sideloading via ADE or Calibre (or other?)?
If the device is secondhand, was it deauthorised from the previous users Adobe ID? Connect the device to the computer, open ADE and hit Ctrl-Shift-E. Then disconnect it and when you plug it back in and start ADE, ADE should ask if you want to authorise it.
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