An authentication agent is used to make the user of a session prove that the user of the session really is the user (by authenticating as the user) or an administrative user (by authenticating as an administrator). The polkit package contains a textual authentication agent called ‘pkttyagent’, which is used as a general fallback.
If you are using a graphical environment, make sure that a graphical authentication agent is installed and autostarted on login.
Cinnamon, Deepin, GNOME, GNOME Flashback, KDE, LXDE, LXQt, MATE, theShell and Xfce have an authentication agent already. In other desktop environments, you have to choose one of the following implementations:
lxqt-policykit, which provides /usr/bin/lxqt-policykit-agent
lxsession, which provides /usr/bin/lxpolkit
mate-polkit, which provides /usr/lib/mate-polkit/polkit-mate-authentication-agent-1
polkit-efl-git, which provides /usr/bin/polkit-efl-authentication-agent-1
polkit-gnome, which provides /usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1
polkit-kde-agent, which provides /usr/lib/polkit-kde-authentication-agent-1
ts-polkitagent, which provides /usr/lib/ts-polkitagent
xfce-polkit-git, which provides /usr/lib/xfce-polkit/xfce-polkit