Welcome to the forums. I have not experienced this issue in 18.04.3 however I have seen this issue referenced to a possible issue with the hardware and ACPI.
Can you open a terminal and run this command shutdown -h now does that shutdown the system? Does it show any errors in the terminal?
You can take a look at this post which is for version 16.04 but still applies to some system today:
The most common fix seems to be:
Make a backup of /etc/default/grub then
Open /etc/default/grub using sudo with your editor of choice.
Edit GRUB_CMDLINE_LINUX_DEFAULT="quite splash" so that it says
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force"
Save and exit your editor.
sudo update-grub
Reboot and then see if you can shutdown correctly.
There are also some options options mentioned in the thread linked above, if this does not work.
Good luck.