Howto Install NextCloud 20 on Ubuntu 20.04.1 The classic way [Wiki]

It does the same as the Web Installer. It generates an instance ID and a basic config.php file based on the information you enter.

I have not read the complete tutorial. But if you can’t connect to the server or it can’t find the page, there is probably something wrong with the configuration of the web server or you have some network/firewall problem. Hard to say without more details.