systemctl: command not found on ubuntu 16.04

when I tried to install RocketChat in Ubuntu 16.04

In Command Prompt I enter.

systemctl enable mongod

After That I got The issue.

systemctl: command not found

How to solve this?

1. First you need to check if systemd package is installed - sudo dpkg -l | grep systemd.
2. If not then install it by hands sudo apt-get install systemd. But if it does it might be damaged, so you may try to reinstall it sudo apt-get install --reinstall systemd.
3. If the package is installed, even after reinstallation it does not work, list the full path of the files inside this package sudo dpkg -L systemd. Maybe binary files are located in a directory that is not included into $PATH variable.