Hi @weavermjordan. Thanks for being part of GoDaddy Community! It's hard to say for sure what's happening in your case. A few things stand out though. First, if you intend to use Installatron to install WordPress, you wouldn't need to create a new database. That would be handled by the installation process. However, that wouldn't usually cause a problem, it's just unnecessary.
Second, if the second domain is registered with GoDaddy and in the same account, adding the domain to the cPanel account should automatically update it's DNS to point to the right server. However, if you have the domain through another provider or in a different GoDaddy account, you'll need to change your DNS manually. That might be the issue at hand since the domain isn't loading a site at all.
Also, to be extra clear, the second domain would need to be registered. I'm guessing the link you shared with mysite.io isn't what you're actually using (though it does show the error you mentioned).
JesseW - GoDaddy | Community Manager | 24/7 support available at x.co/247support | Remember to choose a solution and give kudos.