knocker: 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/knocker - from package: knocker

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

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