ngrep: 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 CentOS possible default full paths are:

/usr/sbin/ngrep - from package: ngrep

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ngrep - from package: ngrep

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

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