volume_key: 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/volume_key - from package: volume_key

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

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