In my case, changing the product name seemed to help, although I'm not completely sure that was the problem as I changed a lot of things while trying to make it work. (the name was starting with a number and I removed it on debug target)

This is still happening on Mac OS Big Sur and Xcode 12.2 stable I cannot preview any swift ui view in my application, and it was working just fine before the update. Does anyone have any suggestions?