numdiff: 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/numdiff - from package: numdiff

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

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