Administrator rights for my original user account restored after being lost

I had problems:

1. I mistakingly deleted the administrator rights from my original user account.
2. Consequently, I could not unlock my users & group preferences and could not install new applications on my iMac using the name and the password from my original account.
3. In particular, I kept getting this message "No such file" when running rm AppleSetupDone from single-user mode.

Here is the solution well described here https://apple.stackexchange.com/questions/347816/no-such-file-when-running-rm-applesetupdone-from-single-user-mode, here There is no administrator account on my m… - Apple Community and here https://apple.stackexchange.com/questions/347816/no-such-file-when-running-rm-applesetupdone-from-single-user-mode

In my case the problem was in the symbols " /" in the command "/sbin/mount -uw /" which was written wrong way "/sbin/mount -uw" (without " /" in the end) in the initial instruction here: I can not unlock my users & group prefere… - Apple Community