6tunnel: 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/6tunnel - from package: 6tunnel

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

Error in other languages:
6tunnel: bevel nie gevind nie
6tunnel: comandă negăsită
6tunnel: comando não encontrado
6tunnel: commande introuvable
6tunnel: command not found
6tunnel: käsku ei ole
6tunnel: không tìm thấy lệnh
6tunnel: komanda nerasta
6tunnel: Kommando nicht gefunden.
6tunnel: kommandot finns inte
6tunnel: komut yok
6tunnel: nie znaleziono polecenia
6tunnel: níor aimsíodh an t-ordú
6tunnel: no se encontró la orden
6tunnel: no s'ha trobat l'ordre
6tunnel: opdracht niet gevonden
6tunnel: parancs nem található
6tunnel: perintah tidak ditemukan
6tunnel: príkaz nenájdený
6tunnel: příkaz nenalezen
6tunnel: команда не найдена
6tunnel: командата не е открита
6tunnel: 命令找不到
6tunnel: コマンドが見つかりません