Creating a Self-Signed Public Key Infrastructure (PKI) for certificate creation [SSL, OpenVPN]

There is a difference in instructions in what is done and the instructions. Use "Key Encipherment" in the 'Key Usage' tab, for this and also for the site certificate creation instead of what is in the document which is "Key Agreement".

how to design application like this one ??

For those wondering if "i12breto" can by replaced with your own name : yes, it can.
You can also place your own location data in the proper fields.
It was probably such obvious for the i12breto that he has forgotten to mention it.
It was not so obvious for me and I had to drink one more coffee to solve it 🙈
Shame, I know. Anyway, the whole procedure works like a charm.
I am on Windows 11 23H2 and XCA v.2.6.0, during the process of installing OpenVPN server in OpenWRT on Linksys WRT1900ACS router. Quite familiar with computers but not much with cryptography so I just clicked through the list more like a monkey. And it worked perfectly.
Many thanks for your time and effort i12breto 👍