As far as I remember, on previous Kubuntu version (18.10) when I hit Ctrl-F9 - KWin shows all open windows. But after upgrade to 19.04 it does not work anymore. In hotkeys settings Ctrl-F9 is still shortcut for this feature, but when I press it - nothing happens and I don't know neither how to launch it from command line to see errors nor where to see logs.
Present Windows is a desktop effect. For it, and certain other desktop effects, to work, compositing needs to be enabled. You can check the status by opening a terminal and running:
qdbus org.kde.KWin /KWin supportInformation | grep -i composit
You may see something like this as the output:
Composite: yes; Version: 0x4 compositingMode: 1 useCompositing: true compositingInitialized: true windowsBlockCompositing: true Compositing Compositing is active Compositing Type: OpenGL
You can toggle compositing on and off using Shift+Alt+F12.
Alternatively, use the GUI to ensure that compositing is turned on when the system starts: