ettercap: 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/ettercap - from package: ettercap

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

On Debian possible default full paths are:

/usr/bin/ettercap - from package: ettercap-graphical
/usr/bin/ettercap - from package: ettercap-text-only

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

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