Hey there! This is almost certainly an issue with the DNS configuration of the hostname or a problem with Apache on the system. The DCV check file gets placed in /var/www/html/.well-known/pki-validation/ - if you don't see a file there already you can create a file named test.txt and see if you can reach it at the following URL:
If that file loads, I would expect the DCV test to pass and the SSL to be issued. If the file doesn't load, you can troubleshoot from there based on the issue - a problem with DNS, issues with port 80, redirection on the hostname, or other issues you may find.
Can you see if you're able to access files from that directory on the server?