ns6: 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/sbin/ns6 - from package: ipv6toolkit

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

Error in other languages:
ns6: bevel nie gevind nie
ns6: comandă negăsită
ns6: comando não encontrado
ns6: commande introuvable
ns6: command not found
ns6: käsku ei ole
ns6: không tìm thấy lệnh
ns6: komanda nerasta
ns6: Kommando nicht gefunden.
ns6: kommandot finns inte
ns6: komut yok
ns6: nie znaleziono polecenia
ns6: níor aimsíodh an t-ordú
ns6: no se encontró la orden
ns6: no s'ha trobat l'ordre
ns6: opdracht niet gevonden
ns6: parancs nem található
ns6: perintah tidak ditemukan
ns6: príkaz nenájdený
ns6: příkaz nenalezen
ns6: команда не найдена
ns6: командата не е открита
ns6: 命令找不到
ns6: コマンドが見つかりません