A stupid question on Letsencrypt using http challenger, "Communication with the API failed, is NPM running correctly?"

[chenzhendong]

When the application is up, I can visit the welcome page of nginx-proxy-manager like

Congratulations!
You've successfully started the Nginx Proxy Manager.
If you're seeing this site then you're trying to access a host that isn't set up yet.
Log in to the Admin panel to get started.

I'd like to apply a ssl cert for my domain on Menu " SSL Certificates" => "Add SSL Certificates". when I click “Test Server Reachability”. It reports "Communication with the API failed, is NPM running correctly?"

My question is: If I can see the welcome page, it proves my connectivity to http://:80 is ok. Why the Reachability test failed for cerbot to get the http challenge from 80 port?

Thanks