poweroff: 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/poweroff - from package: upstart
/usr/bin/poweroff - from package: usermode

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

On Debian possible default full paths are:

/sbin/poweroff - from package: systemd-sysv
/sbin/poweroff - from package: sysvinit-core
/sbin/poweroff - from package: upstart
/usr/lib/klibc/bin/poweroff - from package: klibc-utils
/usr/sbin/poweroff - from package: molly-guard

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

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