I discovered that you must have a BIOS profile that is showing your hack as in the range of computers that will be updatable. so even though I used SWU Switcher Apple wouldn't let me see Big Sur because me computer looked like it could not install it. I had a profile that was for iMac early 2013 and I could not see Big Sur. when I changed it to the correct range then I could see Big Sur and also I see the beta version that was made available the other day in the App store. Time to try is install.
The entry in the boot plist <string>LastBootedVolume</string> needs to be changed to match the name of the drive your boot is on. Easy way to do this is to copy the name of your boot drive from the desktop and paste it in to replace the LastBootedVolume in the text. in my case my drive is called MacOS so I changed the plist from <string>LastBootedVolume</string> to <string>MacOS</string> and this fixed the problem You can do this in Clover Configurator or text edit or other text file editor and save your plist. I always save the plist that works and change the name so I can recover if it does not work.