knock: 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/knock - from package: knockd

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

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