I've run into the same issue on upgrading from Stretch to Buster. After upgrade everything worked and even reboots went fine.
then I decided to upgrade from Buster to Bullseye. When I did that I did an update and upgrade of packages to prepare for the OS upgrade. When I rebooted from that process I got the error above. I'm completely stuck. I'm at the grub rescue prompt and I have no idea how to proceed from here. I've done searches everywhere I can think to look and there seems no definitive way to recover from this. What do I need to do to recover?
