MP4Client: 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/MP4Client - from package: gpac

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

Error in other languages:
MP4Client: bevel nie gevind nie
MP4Client: comandă negăsită
MP4Client: comando não encontrado
MP4Client: commande introuvable
MP4Client: command not found
MP4Client: käsku ei ole
MP4Client: không tìm thấy lệnh
MP4Client: komanda nerasta
MP4Client: Kommando nicht gefunden.
MP4Client: kommandot finns inte
MP4Client: komut yok
MP4Client: nie znaleziono polecenia
MP4Client: níor aimsíodh an t-ordú
MP4Client: no se encontró la orden
MP4Client: no s'ha trobat l'ordre
MP4Client: opdracht niet gevonden
MP4Client: parancs nem található
MP4Client: perintah tidak ditemukan
MP4Client: príkaz nenájdený
MP4Client: příkaz nenalezen
MP4Client: команда не найдена
MP4Client: командата не е открита
MP4Client: 命令找不到
MP4Client: コマンドが見つかりません