duck: 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/duck - from package: duck

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

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