Are you sure you have the latest version of Bitcoin Core from the official website? It sounds like your Bitcoin Core version is out of date and doesn't work on Big Sur.
1. Download the Mac OS version of bitcoin core from
https://bitcoin.org/bin/bitcoin-core-0.20.1/bitcoin-0.20.1-osx.dmg2. Download the checksums used for verifying downloads:
https://bitcoin.org/bin/bitcoin-core-0.20.1/SHA256SUMS.asc3. Open a Terminal and go to the folder that you downloaded Bitcoin Core in (should be in Downloads or something) and run
sha256sum bitcoin-0.20.1-osx.dmg. Then open the SHA256SUMS.asc file, and make sure the checksum on the line
bitcoin-0.20.1-osx.dmg matches the checksum on the Terminal.
If all is good and they match then you are using an authentic Bitcoin Core binary.
If the hashes don't match then you might have downloaded malware and you need to download again from bitcoin.org.Then you can run it and it should show you your funds again.