That's not really an error message, it's just an alert saying that the installer couldn't patch your default airport vehicles (to get rid of the default pushback truck).
A possible reason for this, is that your default airport vehicles file is not original anymore, so the installer simply told you it couldn't be patched, but that's not a problem, the only issue it will cause, is that you might still see the default pushback truck, but GSX will work normally.
