In order to complete this challenge, you need to make your firewall (or your hosting provider’s firewall) allow incoming connections to your server on TCP port 80.
thanks for your suggestion, but i already open the port.
I verified it's not a firewall issue because i set up an expressJS server listening to port 80, and it works. When i run the challenge while the web server is running, i correctly get the error:
problem binding to port 80: Could not bind to IPv4 or IPv6.
My doubt is: is it possible that the port results closed as long as a service is not listening in it? In such case though i'd not be able to get the certificate in standalone, i should switch to a server challenge?