FontDoctor is trying to create a new folder or is trying to access a folder that it does not have permission or access to.
1. From the Desktop, double-click the start-up hard drive icon.
2. Use the "Go" menu and select the "Go to Folder..." menu item, paste this into the field: ~/Library/Application Support/FontDoctor
3. Confirm that the folder exists
2. If it does NOT exist, create it. If it DOES exist, delete it, then recreate it.
3. Launch FontDoctor