Plesk updates fail to install: Failed to retrieve unit: No such method GetUnit

Symptoms

• Plesk installed on Ubuntu 16.04 fails to install updates. The following error can be found in /var/log/plesk/install/autoinstaller3.log :

  CONFIG_TEXT: Starting sw_engine service... Failed to retrieve unit: No such method 'GetUnit'
  done
  Starting sw_cp_server service... Failed to retrieve unit: No such method 'GetUnit'
  done
  ...

• systemctl tool does not working itself:

  # systemctl
  Failed to list units: No such method 'ListUnitsFiltered'

• The server is started with upstart :

  # ps -p 1
  PID TTY          TIME CMD
  1 ?        00:00:02 upstart

Cause

SystemD is required for Plesk on Ubuntu 16.04 to work properly, as /sbin/initctl is not present on the default installation of Ubuntu 16.04

Resolution

1. Connect to the server using SSH
2. Install systemd as follows:

   # apt-get install  systemd-sysv ubuntu-standard

3. Reboot the server
4. Try to install Plesk updates via Tools & Settings > Updates and Upgrades or with the following command:

   # plesk installer --select-release-current --upgrade-installed-components