ip6tables-converter: 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/ip6tables-converter - from package: iptables-converter

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

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