DIAGNOSED AND SOLVED. The reason this happens, is that somehow both Catalina and Big Sur, were somehow changing administrator users, so that they no longer had administrator level clearance. You would end up with not a single administrator on your computer. I tried every solution out there, but they were all useless, until this one. In my case, my internal drive is named "Macintosh HD." If yours is named something else, put that name in where I have written "Macintosh HD," in the following terminal command. First step. Reboot into Recovery mode, by holding down the Command and R keys. You'll see the four options. Instead go up to the menu and go to terminal. In terminal type the following (be sure and type a space after rm) rm “/Volumes/Macintosh HD/var/db/.applesetupdone.” then type reboot You will then be able to create a new user and then enable your existing account to be able to administer the computer. This works. The first time I tried it, after I picked the names and passwords, it said "creating profile." It ran for a couple of hours, without doing anything. I just did a hard reboot and tried it again. This time it worked.