duff: 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/duff - from package: duff

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

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