Hi.
How is this one going? I'm bit late and I hope it got working for you.

Anyways, this is not a error, as you see it's a warn. That mean it warnings you about potential problem but Apache should be working.

What I recommend you to do is to run config tests.
If you run:

sudo apachectl configtest

it'll test your Apache configuration for syntax errors.

Example output of configtest
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message
Syntax OK

It does show warning, but syntax is OK and Apache is working well.

Verify that your site is enabled. You can list all enabled sites with ls:

sudo ls -l /etc/apache2/sites-enabled/

Or try to enable site again with sudo a2ensite site-name.