knockd: 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/knockd - from package: knockd

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

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