fdnsip6list: 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/fdnsip6list - from package: firedns

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

Error in other languages:
fdnsip6list: bevel nie gevind nie
fdnsip6list: comandă negăsită
fdnsip6list: comando não encontrado
fdnsip6list: commande introuvable
fdnsip6list: command not found
fdnsip6list: käsku ei ole
fdnsip6list: không tìm thấy lệnh
fdnsip6list: komanda nerasta
fdnsip6list: Kommando nicht gefunden.
fdnsip6list: kommandot finns inte
fdnsip6list: komut yok
fdnsip6list: nie znaleziono polecenia
fdnsip6list: níor aimsíodh an t-ordú
fdnsip6list: no se encontró la orden
fdnsip6list: no s'ha trobat l'ordre
fdnsip6list: opdracht niet gevonden
fdnsip6list: parancs nem található
fdnsip6list: perintah tidak ditemukan
fdnsip6list: príkaz nenájdený
fdnsip6list: příkaz nenalezen
fdnsip6list: команда не найдена
fdnsip6list: командата не е открита
fdnsip6list: 命令找不到
fdnsip6list: コマンドが見つかりません