rtmux: 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/rtmux - from package: autossh

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

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