Action org.freedesktop.systemtoolsbackends.set is not registered
! pode estar faltando o arquivo .policy
declarando esta ação do polkit
Seu comando funciona bem na máquina (Ubuntu 16.04), vamos acompanhá-lo:
~$ pkcheck --action-id org.freedesktop.systemtoolsbackends.set --allow-user-interaction --process $(pidof -s bash)
polkitretains_authorization_after_challenge=true
polkittemporary_authorization_id=tmpauthz1
~$ grep -r "org.freedesktop.systemtoolsbackends.set" /usr/share/polkit-1/actions/
/usr/share/polkit-1/actions/org.freedesktop.SystemToolsBackends.policy: <action id="org.freedesktop.systemtoolsbackends.set">
~$ dpkg -S /usr/share/polkit-1/actions/org.freedesktop.SystemToolsBackends.policy
system-tools-backends: /usr/share/polkit-1/actions/org.freedesktop.SystemToolsBackends.policy
Para confirmar o problema, verifique se você o instalou usando apt-cache policy system-tools-backends
Para corrigir, tente reinstalar o pacote:
sudo apt-get install --reinstall system-tools-backends