dff: 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/dff - from package: dff

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

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