etterfilter: 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/bin/etterfilter - from package: ettercap

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

On Debian possible default full paths are:

/usr/bin/etterfilter - from package: ettercap-common

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

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