addr6: 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/addr6 - from package: ipv6toolkit

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

Error in other languages:
addr6: bevel nie gevind nie
addr6: comandă negăsită
addr6: comando não encontrado
addr6: commande introuvable
addr6: command not found
addr6: käsku ei ole
addr6: không tìm thấy lệnh
addr6: komanda nerasta
addr6: Kommando nicht gefunden.
addr6: kommandot finns inte
addr6: komut yok
addr6: nie znaleziono polecenia
addr6: níor aimsíodh an t-ordú
addr6: no se encontró la orden
addr6: no s'ha trobat l'ordre
addr6: opdracht niet gevonden
addr6: parancs nem található
addr6: perintah tidak ditemukan
addr6: príkaz nenájdený
addr6: příkaz nenalezen
addr6: команда не найдена
addr6: командата не е открита
addr6: 命令找不到
addr6: コマンドが見つかりません