In honesty, you probably need to build some more familiarity around the core of a LAMP (Linux, Apache, MariaDB, and PHP) stack (and optionally Certbot/ACME and Git) before deploying ITFlow.
You don't have to be an expert by any means! My Linux skills aren't the best, but you should really be comfortable navigating the CLI, installing programs, and reading error logs to self-host any PHP app. As ITFlow is intended for use by other IT professionals, we do (even unintentionally) assume you have an OK working knowledge of Linux.
I've not used the install script myself, but it seems to be complaining about a virtual host not defined for port 80. Let's Encrypt relies on this to confirm that your A record is correct and to verify the certificate request. The default location for your virtual hosts is /etc/apache2/sites-available/with the default file named 000-default.conf.