nngrep: 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/nngrep - from package: nn

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

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