shutdown: 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 CentOS possible default full paths are:

/sbin/shutdown - from package: upstart

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/shutdown - from package: systemd-sysv
/sbin/shutdown - from package: sysvinit-core
/sbin/shutdown - from package: upstart
/usr/sbin/shutdown - from package: molly-guard

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

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