zshutdown_notify: 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:

/usr/sbin/zshutdown_notify - from package: zephyr-clients

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

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