ndptool: 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/ndptool - from package: libndp-tools

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

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