Do you see your Chromecast devices appear at all under Roon Settings -> Displays?
What is the model of the Chromecast gear you’re using?
Have you checked your Windows Firewall to make sure that Roon is an exception?
Since you are using a Windows PC, I would verify that both Roon.exe and RAATServer.exe have been added as exceptions to your Windows firewall.
You can use these instructions to add the exceptions and the executables themselves would be located in your Database Location/Application folder path.
I would also add these exceptions to any Antivirus or other Firewall blocking applications you may have and ensure that you connected to your network via a Private network, not a Public one, see this guide for more information.
Wow…That was simple. It may need to be added to the online documentation. Either I missed it (entirely possible) or it’s not in the place it needs to be when someone is referring to the docs.