Why can't I install Xcode 4.4 on Mountain Lion?

Solution 1:

You can manually clear the store where the App Store saves the downloads before completion:

  1. Quit the App Store app
  2. Open Terminal.app (search with Spotlight)
  3. rm -r /private/var/folders/*/*/*/com.apple.appstore

    This step is the removal part...

  4. Restart the App store.app