Hello @Stu_Farnham,
We have not yet updated Roon to support MacOS Mojave. Stay tuned for official support closer to the official release date of Mojave.
Head to System Preferences > Security & Privacy, Privacy Tab, then scroll to the Accessibility list. Here, click the plus button and add Roon.app to the allowed list. You may need to click the lock in the lower left corner to allow changes to this screen.
If you already see Roon in this list, highlight it and click the ‘-’ button to get rid of this existing entry. Then click the ‘+’ button and add Roon.app to the list again.
-John