MP4Box: 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/MP4Box - from package: gpac

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

Error in other languages:
MP4Box: bevel nie gevind nie
MP4Box: comandă negăsită
MP4Box: comando não encontrado
MP4Box: commande introuvable
MP4Box: command not found
MP4Box: käsku ei ole
MP4Box: không tìm thấy lệnh
MP4Box: komanda nerasta
MP4Box: Kommando nicht gefunden.
MP4Box: kommandot finns inte
MP4Box: komut yok
MP4Box: nie znaleziono polecenia
MP4Box: níor aimsíodh an t-ordú
MP4Box: no se encontró la orden
MP4Box: no s'ha trobat l'ordre
MP4Box: opdracht niet gevonden
MP4Box: parancs nem található
MP4Box: perintah tidak ditemukan
MP4Box: príkaz nenájdený
MP4Box: příkaz nenalezen
MP4Box: команда не найдена
MP4Box: командата не е открита
MP4Box: 命令找不到
MP4Box: コマンドが見つかりません