stun-server: 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:

/usr/sbin/stun-server - from package: stun-server

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

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