mp4tags: 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/mp4tags - from package: libmp4v2

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

On Debian possible default full paths are:

/usr/bin/mp4tags - from package: mp4v2-utils

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

Error in other languages:
mp4tags: bevel nie gevind nie
mp4tags: comandă negăsită
mp4tags: comando não encontrado
mp4tags: commande introuvable
mp4tags: command not found
mp4tags: käsku ei ole
mp4tags: không tìm thấy lệnh
mp4tags: komanda nerasta
mp4tags: Kommando nicht gefunden.
mp4tags: kommandot finns inte
mp4tags: komut yok
mp4tags: nie znaleziono polecenia
mp4tags: níor aimsíodh an t-ordú
mp4tags: no se encontró la orden
mp4tags: no s'ha trobat l'ordre
mp4tags: opdracht niet gevonden
mp4tags: parancs nem található
mp4tags: perintah tidak ditemukan
mp4tags: príkaz nenájdený
mp4tags: příkaz nenalezen
mp4tags: команда не найдена
mp4tags: командата не е открита
mp4tags: 命令找不到
mp4tags: コマンドが見つかりません