arno-fwfilter: 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/arno-fwfilter - from package: arno-iptables-firewall

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

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