Hello,
I encountered the same problem as you when building on the new Big Sur.
My device: Mac16, base model with latest OS update.
I am building a mini version of xhyve.
My workaround was that I compiled with the following flag(assuming you use clang): -mmacosx-version-min=10.10
You can also select this target if you build on xcode.
Now it is running for me.