rd6: 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/rd6 - from package: ipv6toolkit

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

Error in other languages:
rd6: bevel nie gevind nie
rd6: comandă negăsită
rd6: comando não encontrado
rd6: commande introuvable
rd6: command not found
rd6: käsku ei ole
rd6: không tìm thấy lệnh
rd6: komanda nerasta
rd6: Kommando nicht gefunden.
rd6: kommandot finns inte
rd6: komut yok
rd6: nie znaleziono polecenia
rd6: níor aimsíodh an t-ordú
rd6: no se encontró la orden
rd6: no s'ha trobat l'ordre
rd6: opdracht niet gevonden
rd6: parancs nem található
rd6: perintah tidak ditemukan
rd6: príkaz nenájdený
rd6: příkaz nenalezen
rd6: команда не найдена
rd6: командата не е открита
rd6: 命令找不到
rd6: コマンドが見つかりません