You are installing CentOS 8.x but when you click on the "Installation Source" tab, nothing happens. You do not see an option to enter an installation source.
You cannot click "Begin installation." What do you do?
Try different installation media. If you are using a boot.iso, this problem may happen.
Some DVD isos are here: http://ftp.hosteurope.de/mirror/centos.org/8/isos/x86_64/