The error message 'A valid copy of QuarkXPress is already installed on this machine.' can be resolved by trying below steps:
- Close error message pop-up
- Quit QuarkXPress application.
- Navigated to Mac HDD/Users/Home User /Library/Application Support/Quark/QuarkXPress
- Trash com.quark.quarkxpress 9.setup.plist file
- Re-install QuarkXPress application.
If you have further issues please feel free to contact http://support.quark.com