mplex: 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/mplex - from package: mjpegtools

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

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