umshutdown: command not found

Have You tried accessing the binary with the full path?

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

[root@server ~]# echo $PATH

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/bin/umshutdown - from package: umview

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