nfnl_osf: 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/nfnl_osf - from package: iptables

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

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