I have the same problem.
When I first opened the creator tool, the usb stick was shown with GUID and everything was fine. I didn't flash it and restarted the program later. Then the same stick, without disconnecting it between the two program starts, was shown with MISSING GUID.
From there on, I never got the Stick to work again with the creator tool. Changing USB-Port, restarting the PC, reinstalling the software, nothing helped.
It's a shame, that they can not make it work correctly.