systemctl: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/bin/systemctl - from package: systemd

You may install the required package with command apt-get install {package}

Error in other languages:
systemctl: bevel nie gevind nie
systemctl: comandă negăsită
systemctl: comando não encontrado
systemctl: commande introuvable
systemctl: command not found
systemctl: käsku ei ole
systemctl: không tìm thấy lệnh
systemctl: komanda nerasta
systemctl: Kommando nicht gefunden.
systemctl: kommandot finns inte
systemctl: komut yok
systemctl: nie znaleziono polecenia
systemctl: níor aimsíodh an t-ordú
systemctl: no se encontró la orden
systemctl: no s'ha trobat l'ordre
systemctl: opdracht niet gevonden
systemctl: parancs nem található
systemctl: perintah tidak ditemukan
systemctl: príkaz nenájdený
systemctl: příkaz nenalezen
systemctl: команда не найдена
systemctl: командата не е открита
systemctl: 命令找不到
systemctl: コマンドが見つかりません