ip6tables: 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:

/sbin/ip6tables - from package: iptables

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

Error in other languages:
ip6tables: bevel nie gevind nie
ip6tables: comandă negăsită
ip6tables: comando não encontrado
ip6tables: commande introuvable
ip6tables: command not found
ip6tables: käsku ei ole
ip6tables: không tìm thấy lệnh
ip6tables: komanda nerasta
ip6tables: Kommando nicht gefunden.
ip6tables: kommandot finns inte
ip6tables: komut yok
ip6tables: nie znaleziono polecenia
ip6tables: níor aimsíodh an t-ordú
ip6tables: no se encontró la orden
ip6tables: no s'ha trobat l'ordre
ip6tables: opdracht niet gevonden
ip6tables: parancs nem található
ip6tables: perintah tidak ditemukan
ip6tables: príkaz nenájdený
ip6tables: příkaz nenalezen
ip6tables: команда не найдена
ip6tables: командата не е открита
ip6tables: 命令找不到
ip6tables: コマンドが見つかりません