In my experiments I was inaccurate enough to untick

"enable administrator privileges for this user"

checkmark on my only system user.

Now I cannot set in back.

How can I login as root to give back privileges to my user ? There is no user choose option at login screen.

Thanks!

which distribution?

You should boot from a live-medium and chroot into the installed system as root if your current root-account is passwordless.

1.
My kubuntu version
> % uname -a
Linux kubuntu-dell 5.3.0-51-generic #44~18.04.2-Ubuntu SMP Thu Apr 23 14:27:18 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Should have mention it earlier, my bad

2.
I have never used chroot before. Made some googling, sounds like something I can do:
https://www.turnkeylinux.org/docs/chroo ... air-system
What do I do after I`m in?
Edit passwd ?
Or probably I shold better create new user with privileges?

3.
I am not sure if my root passwordless or not.
What is the default password if I never changed it? Empty?

mostly-harmless wrote:2.

Re-add the user to the sudoers group with usermod -aG sudo YOUR_USERNAME

3.

The default password in Ubuntu is not set, so there is no root login allowed. (You may switch to a root shell with sudo -i or sudo -s anyways)