Please try following steps:
1. Close Elements Organizer
2. Go to Task Manager and look for the following exes and kill them:
(If you are using Mac then please open Activity Monitor and kill the above-mentioned processes.)
3. Now relaunch Organizer and try creating Slideshow again.
If it does not work, please reboot your system and then launch Elements Organizer.Please revert if you still face any issue.