Tails Installer makes use of policykit-1, that allows running the program as
non-privileged user while asking for a root password only for the operations
that need it: writing the MBR and unmounting the USB stick once the
installation is finished.

When running minimalistic desktop environments this does not work out of the
box. `/usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1` must be
running, or the program needs to be executed as root.

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814190