Since i have no access to my root account on my newly installed pureos 8.0

how can i add a regular user to sudoer list ?

How can i do anything with command line ?

If you need to change the root password:

Open Terminal and issue following command:

sudo su

su is the command to change user (you can simply run man su in
terminal to see how the manual). This will ask your password, enter it,
and you are now a root user. Then issue following command:

passwd

You will be asked to “enter new UNIX password”, twice. You will get no
feedback when typing your password, don’t let that confuse you.

Here’s a good start: https://www.debian.org/doc/manuals/debian-reference/ch01.en.html

Mladen
thank you for your patience, caring and invaluable guidance