fwknopd: 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 CentOS possible default full paths are:

/usr/sbin/fwknopd - from package: fwknop

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/fwknopd - from package: fwknop-server

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

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