mlabel: 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/mlabel - from package: mtools

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

On Debian possible default full paths are:

/usr/bin/mlabel - from package: mtools

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

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