netsniff-ng: 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/netsniff-ng - from package: netsniff-ng

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

On Debian possible default full paths are:

/usr/sbin/netsniff-ng - from package: netsniff-ng

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

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