demux: command not found

Have You tried accessing the binary with the full path?

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/demux - from package: gromacs

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