Hi Pete,
By selecting “Use This Mac”, it sound like you’re trying to create a new Core on your Mac. Is the Mac your usual Core or do you use it as Remote?
Support should be along to help. In order for Roon’s @support team to better assist you, please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so they can have a clear understanding of how your devices are communicating.