prepmx: 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/prepmx - from package: m-tx

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

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