ndiff: 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 CentOS possible default full paths are:

/usr/bin/ndiff - from package: nmap

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ndiff - from package: ndiff

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

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