Hi @justatest,
It seems like you might be confusing standalone and webroot. The webroot method involves creating files on your existing webserver (which Certbot should do for you—you don’t have to do it yourself), while the standalone method is a complete alternative to your existing web server, which normally requires you to stop the existing server process while obtaining your certificate.
Right now, there is something listening on http://justatest.com/ on port 80, so if you want to use the standalone method, you would have to shut that down temporarily in order to free up port 80 for Certbot to use.