mcxsubs: 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/mcxsubs - from package: mcl

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

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