fwknop: 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/bin/fwknop - from package: fwknop

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

On Debian possible default full paths are:

/usr/bin/fwknop - from package: fwknop-client

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

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