This error can occur due to a corrupted download or when System Integrity Protection (SIP) settings are interfering. Solution : Verify the downloaded file's integrity using its MD5 checksum. If it's correct, try installing after temporarily disabling SIP in the Recovery Mode.

Complete Guide to Downloading and Creating a macOS Big Sur 11.2.1 Installer DMG

If you are running an unsupported Mac, you may need a patcher tool like Ben's patcher to install Big Sur.