mp3rename: 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/mp3rename - from package: mp3rename

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

Error in other languages:
mp3rename: bevel nie gevind nie
mp3rename: comandă negăsită
mp3rename: comando não encontrado
mp3rename: commande introuvable
mp3rename: command not found
mp3rename: käsku ei ole
mp3rename: không tìm thấy lệnh
mp3rename: komanda nerasta
mp3rename: Kommando nicht gefunden.
mp3rename: kommandot finns inte
mp3rename: komut yok
mp3rename: nie znaleziono polecenia
mp3rename: níor aimsíodh an t-ordú
mp3rename: no se encontró la orden
mp3rename: no s'ha trobat l'ordre
mp3rename: opdracht niet gevonden
mp3rename: parancs nem található
mp3rename: perintah tidak ditemukan
mp3rename: príkaz nenájdený
mp3rename: příkaz nenalezen
mp3rename: команда не найдена
mp3rename: командата не е открита
mp3rename: 命令找不到
mp3rename: コマンドが見つかりません