modplug123: 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/modplug123 - from package: modplug-tools

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

Error in other languages:
modplug123: bevel nie gevind nie
modplug123: comandă negăsită
modplug123: comando não encontrado
modplug123: commande introuvable
modplug123: command not found
modplug123: käsku ei ole
modplug123: không tìm thấy lệnh
modplug123: komanda nerasta
modplug123: Kommando nicht gefunden.
modplug123: kommandot finns inte
modplug123: komut yok
modplug123: nie znaleziono polecenia
modplug123: níor aimsíodh an t-ordú
modplug123: no se encontró la orden
modplug123: no s'ha trobat l'ordre
modplug123: opdracht niet gevonden
modplug123: parancs nem található
modplug123: perintah tidak ditemukan
modplug123: príkaz nenájdený
modplug123: příkaz nenalezen
modplug123: команда не найдена
modplug123: командата не е открита
modplug123: 命令找不到
modplug123: コマンドが見つかりません