I got it.
Just to be sure, that I did everything right, I'll share my steps.
I wrote in my terminal:
echo 'systemd.log_level=debug' | sudo tee /etc/kernel/cmdline
the file was created successfully
then... the very first time I've experienced hanging with the Shutdown process,
after I rebooted, I wrote in the terminal:
sudo journalctl -b -1 > /tmp/journalctl.txt
I hope I did it correctly.
I'm noticing many Errors and Fails. Not sure what they are presenting.
Most noticeable are - ACPI BIOS Error (bug)
and - [sda] tag#(number) FAILED