ndselect: 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/ndselect - from package: numdiff

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

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