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Without sudo, you may be prompted to authenticate as root; in order that the command might be run as root, I typically see this dialogue when I forget to sudo first:

==== AUTHENTICATING FOR org.freedesktop.systemd1.manage-units ===
Authentication is required to manage system services or units.
Authenticating as: root
Password:

You can also confirm whether a service is started or stopped quickly with the status argument:

$ systemctl status postfix
● postfix.service - Postfix Mail Transport Agent
   Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
   Active: inactive (dead) since Sun 2018-08-26 13:43:46 UTC; 2min 56s ago