pkfilter: 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/pkfilter - from package: pktools

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

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