mmg: 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 CentOS possible default full paths are:

/usr/bin/mmg - from package: mkvtoolnix-gui

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mmg - from package: mkvtoolnix-gui

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

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