Comment 2 for bug 1727817

There should be a message like this in the log (or DpkgTerminalLog.gz in the automatic bug report):

usermod: Ο χρήστης "<τάδε>" δεν βρέθηκε

You need to specify a valid username, i.e. the name you use to login.
Could you please purge the package:
  sudo apt-get purge kismet
and then install it again?

For the time being I'm marking the bug as a duplicate, please write here if you cannot resolve the issue.

Kind regards,
Nick