mren: 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/mren - from package: mtools

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

On Debian possible default full paths are:

/usr/bin/mren - from package: mtools

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

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