scan6: 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/scan6 - from package: ipv6toolkit

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

Error in other languages:
scan6: bevel nie gevind nie
scan6: comandă negăsită
scan6: comando não encontrado
scan6: commande introuvable
scan6: command not found
scan6: käsku ei ole
scan6: không tìm thấy lệnh
scan6: komanda nerasta
scan6: Kommando nicht gefunden.
scan6: kommandot finns inte
scan6: komut yok
scan6: nie znaleziono polecenia
scan6: níor aimsíodh an t-ordú
scan6: no se encontró la orden
scan6: no s'ha trobat l'ordre
scan6: opdracht niet gevonden
scan6: parancs nem található
scan6: perintah tidak ditemukan
scan6: príkaz nenájdený
scan6: příkaz nenalezen
scan6: команда не найдена
scan6: командата не е открита
scan6: 命令找不到
scan6: コマンドが見つかりません