Catalina has some issues. Early adopters always discover the bugs before Apple fixes things.
We do have people able to see designs using Thumbnailer on Catalina.
First, make sure you've installed the Thumbnailer currently on our Downloads page. (see attached picture for a picture of the download page showing the Catalina information) www.Embrilliance.com/downloads
New computers running Catalina have not had any trouble if they install this current update, follow the instructions to allow access and reboot the computer.
If you have LOTS of embroidery designs on your computer, you may have to leave the computer on overnight in order for the quick look generator to create thumbnails for all of your files. Be sure to shut down the computer and restart after a few seconds in the morning.
We have some posts to help Apple Catalina users, one is here, describing the Allow Access process:
https://embrilliance.com/archives/2835
You may need to reboot a couple times on the Mac to get QuickLook to work properly.
It's an Apple bug, and there's not much we can do about it, but for many users, it is working, after a bit of tinkering. Here's more information on the issue with Catalina.: https://discussions.apple.com/thread/250724088?answerId=251378020022#251378020022