stun: 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/bin/stun - from package: stun

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

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