nngrab: 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/nngrab - from package: nn

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