dff-gui: command not found

Have You tried accessing the binary with the full path?

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-gui - from package: dff

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