Today's experiment follows.  "OS"=opensnitch, UI=opensnitch-ui 

1. Clean install.  Per KDE System Monitor, OS running. Opened UI via KDE app launcher.  Per KDE System monitor, both OS and UI are now running. PROBLEM: UI window does not open.

2. Stop open processes:

$ sudo service opensnitch stop; pkill -9 opensnitch opensnitch-ui
pkill: only one pattern can be provided
Try `pkill --help' for more information.

$ sudo service opensnitch stop

$ sudo service opensnitch-ui stop
Failed to stop opensnitch-ui.service: Unit opensnitch-ui.service not loaded.

3. Closed UI via System Monitor.  Wait 5 min.  Neither OS or UI running.

4. Open UI via user, as requested:

$ opensnitch-ui
Loading translations: /usr/lib/python3/dist-packages/opensnitch/i18n locale: en_US

PROBLEM: Terminal does not return any message or a prompt.  After 5 min, close window.  
UI also closes per System Monitor.  Neither OS or UI are running.

5. Open OS first:

$ opensnitch
bash: opensnitch: command not found

$ opensnitchd
dwl@x11sca:~$ opensnitchd
[2021-02-14 14:09:21]  IMP  Starting opensnitch-daemon v1.3.5
[2021-02-14 14:09:21]  INF  Loading rules from /home/dwl/rules ...
[2021-02-14 14:09:21]  !!!  Path '/home/dwl/rules' does not exist

8. Open UI via KDE app launcher

UI running per System Monitor, but OS is not running.  UI window does not appear.

9. Logout, login again.  UI is running on login, but OS is not running.

10. Try to close UI, first via sudo then as user

$ sudo service opensnitch-ui stop
Failed to stop opensnitch-ui.service: Unit opensnitch-ui.service not loaded.

$ service opensnitch-ui stop
bash: service: command not found

However, UI is still running per System Monitor.

SUMMARY: On initial install, the UI icon appeared in KDE's panel, and the UI window would open.  The problem was that the UI status was "disabled".  I am not able to re-create that status, even after removing and re-installing the two deb packages.  After killing OS, it cannot be launched from the terminal or from the KDE launcher. UI can be launched, but its window does not appear.