In order to fix the problem, select a video file from the timeline and click Reveal in Finder from the context menu. Once your current project appears in Finder, delete all files within the folders render files and transcoded media, close the application, empty the Trash, launch the program again and try to export. If it doesn't work, go to the Applications directory, select Final Cut Pro X and click Get Info. Check the box that tells you to disable AppNap and try again. If the problem persists, access the official forum for further solution.
