Hi Alex,
That video is not showing how to install PixInsight on Linux, but how to execute it 'locally'. This is not what you want. Do the following to perform a correct installation:
1. Download the Linux installation archive. For the current version, it is:
PI-linux-x64-1.8.8-5-20200228-c.tar.xz
Normally when you download a file it is stored on your Downloads directory. So let's assume that this file is now on your ~/Downloads directory (the '~' is a conventional shortcut for your home folder on UNIX/Linux).
2. Open a terminal. On Kubuntu (or any distribution using KDE for that matter), run the Konsole application (from the 'K' menu).
3. Enter the following commands:
Bash:
cd ~/Downloads
tar -xf PI-linux-x64-1.8.8-5-20200228-c.tar.xz
sudo ./installer
These commands do the following, respectively: change your current directory to Downloads under your home folder, uncompress the installation archive, and execute the installer program as root (root is the conventional name for 'superuser', or 'administrator', on UNIX/Linux).
Now simply answer 'yes' (no quotes), and PixInsight will be installed with default options. After this, you'll find PixInsight on your 'K' menu, just as any application installed on your machine.
Let me know if you need more information.
I've been told that it's faster with Linux.
Indeed it is, and much more stable.