KDE Plasma Version: 5.8.6
KDE Frameworks Version: 5.28.0
QT Version: 5.7.1

When clicking onto the encircled-exclamation-mark-icon located in the taskbar (Notifications), a panel (window) will pop-up indicating "No new notifications", in fact, there are never any notifications displayed in this particular panel (window).

Configuring various Event Sources in Event Notifications and Actions do not make any difference (eg: "Show a message in a popup" and/or "Mark taskbar entry").

How can the system configured to populate the Notification panel?

Edit:-
It seems Plasma Version: 5.8.6 does not have a 'history of notification' function.
There is an 'Full Notifications KDE 5 Applet' available at KDE Store which I added to the Widgets; It works very well.