A little late, but...I just had this problem on Big Sur. I had to power down my laptop because (I'm guessing here) an update was failing to complete. When I restarted, I was asked for a WiFi network, and then got that "your mac is missing an important update". Re-try failed. I rebooted in safe mode (holding down left-shift while restarting) and everything looked normal. I followed the directions for using Software Update to fix the problem..and was told "your mac is up to date!" It had updated during the "safe mode" reboot. I restarted normally, and all appears well. If anyone knows what actually happened, I'd love to know - but right now all seems back to order.