An error occurred preparing the software update

Two things to attempt. Shutdown using Power Button. Restart and immediately hold Shift key. This is Safe Mode and will perform a Repair Disk, clear cache files and only load required Apple Software. It boot slowly - Normal. This may allow the installer to finish and present Big Sur Setup Assist.

If that does not resolve this, again restart and hold Option key immediately and choose Macintosh HD as Startup disk.

Hi!

I have the exact same problem, can't use my computer anymore. Asks for the Macintosh HD password but doesn't recognize my passwords. The target disk mode doesn't help, since it asks the same passwords and doesn't accept them. I'm also the first and only owner of my computer so I'm confused on what passwords the system wants.

Did you find any solutions for this problem yet?

Please see this Apple Support linked file specific to this issue. Once one has worked through their suggestion and if no resolution - am afraid to say one might have to Erase and re-Install from a blank computer. Must mention. have seen others with similar problem and their solution was to Erase and re-install.