ra6: 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/ra6 - from package: ipv6toolkit

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

Error in other languages:
ra6: bevel nie gevind nie
ra6: comandă negăsită
ra6: comando não encontrado
ra6: commande introuvable
ra6: command not found
ra6: käsku ei ole
ra6: không tìm thấy lệnh
ra6: komanda nerasta
ra6: Kommando nicht gefunden.
ra6: kommandot finns inte
ra6: komut yok
ra6: nie znaleziono polecenia
ra6: níor aimsíodh an t-ordú
ra6: no se encontró la orden
ra6: no s'ha trobat l'ordre
ra6: opdracht niet gevonden
ra6: parancs nem található
ra6: perintah tidak ditemukan
ra6: príkaz nenájdený
ra6: příkaz nenalezen
ra6: команда не найдена
ra6: командата не е открита
ra6: 命令找不到
ra6: コマンドが見つかりません