mrename: 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 Debian possible default full paths are:

/usr/bin/mrename - from package: mrename

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

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