As I run vanilla xmonad, I don’t get nice things started for me when I boot up. Though I normally install packages with dpkg -i package.deb, sometimes I want to use the software centre and I need to start PolicyKit by hand.
The error I get starts with org.freedesktop.PolicyKit.Error.Failed, and the command to start policykit is as follows:
/usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1
This is via AskUbuntu, but I can never find it when I need it so I decided to reproduce here.