na6: 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/na6 - from package: ipv6toolkit

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

Error in other languages:
na6: bevel nie gevind nie
na6: comandă negăsită
na6: comando não encontrado
na6: commande introuvable
na6: command not found
na6: käsku ei ole
na6: không tìm thấy lệnh
na6: komanda nerasta
na6: Kommando nicht gefunden.
na6: kommandot finns inte
na6: komut yok
na6: nie znaleziono polecenia
na6: níor aimsíodh an t-ordú
na6: no se encontró la orden
na6: no s'ha trobat l'ordre
na6: opdracht niet gevonden
na6: parancs nem található
na6: perintah tidak ditemukan
na6: príkaz nenájdený
na6: příkaz nenalezen
na6: команда не найдена
na6: командата не е открита
na6: 命令找不到
na6: コマンドが見つかりません