Authentication is required to manage system services or units.
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.
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).