ndpmon: 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/sbin/ndpmon - from package: ndpmon

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

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