ngp2: command not found

Have You tried accessing the binary with the full path?

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/ngp2 - from package: ngraph-gtk

You may install the required package with command apt-get install {package}
Error in other languages:
ngp2: bevel nie gevind nie
ngp2: comandă negăsită
ngp2: comando não encontrado
ngp2: commande introuvable
ngp2: command not found
ngp2: käsku ei ole
ngp2: không tìm thấy lệnh
ngp2: komanda nerasta
ngp2: Kommando nicht gefunden.
ngp2: kommandot finns inte
ngp2: komut yok
ngp2: nie znaleziono polecenia
ngp2: níor aimsíodh an t-ordú
ngp2: no se encontró la orden
ngp2: no s'ha trobat l'ordre
ngp2: opdracht niet gevonden
ngp2: parancs nem található
ngp2: perintah tidak ditemukan
ngp2: príkaz nenájdený
ngp2: příkaz nenalezen
ngp2: команда не найдена
ngp2: командата не е открита
ngp2: 命令找不到
ngp2: コマンドが見つかりません