Hi Andrew,
Are you attempting to install Discourse on a server which already has an Apache2 installation?
Visiting your above URL I see an Apache2 default page, which is going to prevent Discourse from being installed. Discourse is an application, rather than a website, and doesn’t need an external web server. Everything you need to get up and running is contained within the docker image.
The simplest solution would be to uninstall Apache2. If you need it for other purposes I would strongly recommend running through a the recommended install process the first time on a different server.
Running Discourse alongside other web servers and applications is a reasonably advanced topic, and not a great starting point when you’re trying to get to grips with everything else.