I am on a MacBook Pro 2020, and Parallels 16. A similar problem persists in Big Sur's public release.
I encounter an error message too, saying I have to restart my Mac to run my virtual PC. I had to dial back to Catalina in order to run my virtual machine. There is a work-related app I need for work that runs on Windows, and until that is resolved, I cannot upgrade to Big Sur.
DavidStaMaria
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