gnubiff: 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/gnubiff - from package: gnubiff

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

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