Authentication is required to manage system services or units.

Solution 1:

When managing system services, you need to do it as root not as normal user. This is indicated by $ character at the end of shell prompt.

You can either use sudo command or switch to root (root shell usually indicated by # char).

Solution 2:

Your system is using the polkit Authorization Manager and the message is from the file /usr/share/polkit-1/actions/org.freedesktop.systemd1.policy. polkit can be configured in /etc/polkit-1 and /usr/share/polkit-1 directories, more specifically in the rules.d and actions subdirectories. See the Polkit man page for more information.