First you need to check if systemd package is installed –
sudo dpkg -l | grep systemd.
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.
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