mmv: 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/mmv - from package: mmv

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

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