SystemModeler autodetects and uses the latest compiler on your machine. With the release of Xcode 11, SystemModeler will not detect the compiler bundled with the new Xcode version correctly and may return this error message:

...fatal error: 'errno.h' file not found
#include_next 

You can resolve this problem by pointing SystemModeler to a different compiler:

1. Go to Preferences ► Global ► Compiler to configure your macOS SDK path.
   
2. Change the ‘Select Compiler’ to “Custom”.



3. Set the “Path to Mac OS X platform SDK” path to:
   
   /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

   The file path will be different if your Xcode.app is not in your Applications folder.
   
   

4. Click Apply for changes to take effect.
5. Click OK to close the Preferences window.
6. Restart your SystemModeler and re-simulate your model to verify the error message no longer appears.