Either you're using the KDE enabled version, in which case you at least have kdelibs installed or you've got quassel-qt installed and it shouldn't know anything about KNotify. Try quassel-qt if you haven't.
Either you're using the KDE enabled version, in which case you at least have kdelibs installed or you've got quassel-qt installed and it shouldn't know anything about KNotify. Try quassel-qt if you haven't.