mp4subtitle: 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/mp4subtitle - from package: mp4v2-utils

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

Error in other languages:
mp4subtitle: bevel nie gevind nie
mp4subtitle: comandă negăsită
mp4subtitle: comando não encontrado
mp4subtitle: commande introuvable
mp4subtitle: command not found
mp4subtitle: käsku ei ole
mp4subtitle: không tìm thấy lệnh
mp4subtitle: komanda nerasta
mp4subtitle: Kommando nicht gefunden.
mp4subtitle: kommandot finns inte
mp4subtitle: komut yok
mp4subtitle: nie znaleziono polecenia
mp4subtitle: níor aimsíodh an t-ordú
mp4subtitle: no se encontró la orden
mp4subtitle: no s'ha trobat l'ordre
mp4subtitle: opdracht niet gevonden
mp4subtitle: parancs nem található
mp4subtitle: perintah tidak ditemukan
mp4subtitle: príkaz nenájdený
mp4subtitle: příkaz nenalezen
mp4subtitle: команда не найдена
mp4subtitle: командата не е открита
mp4subtitle: 命令找不到
mp4subtitle: コマンドが見つかりません