Starting with MacOS Mojave and made more strict in Catalina,  MacOS requires that applications be granted permissions to access the user's Documents, Downloads and Desktop folders as well as external and network drives.  While some of these permissions are set during install or will be requested the first time the Renderer accesses them by opening, exporting or creating a new master file, the MP4 export feature requires the proper permissions to access folders and drives before the export process.    

 

In Catalina this is done by accessing the folders with “Open Master File” option from Renderer File menu or making sure that a given folder has permissions from System Preferences/Security & Privacy.  Click on the "Privacy" tab and then "Files and Folders" in the left pane and select the folders needed for access by the Dolby Atmos Renderer in the right pane as pictured below.