Hi!
BigSur has many compatibility issues, check this post and see if any of the solutions work for you:

https://forums.virtualbox.org/viewtopic.php?f=39&t=98763&start=15

Also check the link below:

https://medium.com/@Aenon/mac-virtualbox-kernel-driver-error-df39e7e10cd8

Let me know how it goes!
Diego