fdnsip6: 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/fdnsip6 - from package: firedns

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

Error in other languages:
fdnsip6: bevel nie gevind nie
fdnsip6: comandă negăsită
fdnsip6: comando não encontrado
fdnsip6: commande introuvable
fdnsip6: command not found
fdnsip6: käsku ei ole
fdnsip6: không tìm thấy lệnh
fdnsip6: komanda nerasta
fdnsip6: Kommando nicht gefunden.
fdnsip6: kommandot finns inte
fdnsip6: komut yok
fdnsip6: nie znaleziono polecenia
fdnsip6: níor aimsíodh an t-ordú
fdnsip6: no se encontró la orden
fdnsip6: no s'ha trobat l'ordre
fdnsip6: opdracht niet gevonden
fdnsip6: parancs nem található
fdnsip6: perintah tidak ditemukan
fdnsip6: príkaz nenájdený
fdnsip6: příkaz nenalezen
fdnsip6: команда не найдена
fdnsip6: командата не е открита
fdnsip6: 命令找不到
fdnsip6: コマンドが見つかりません