ettercap-pkexec: command not found

Have You tried accessing the binary with the full path?

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/ettercap-pkexec - from package: ettercap-graphical

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