stun-client: 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/bin/stun-client - from package: stun

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

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