The apps automatically detect if a GUI environment is present and load the desktop version automatically. If not present, they default to the command-line or headless version. Apparently, I also need to check if a notification area in the desktop also exists. After I added the applet, both apps ran fine.
21 September 2022
NetCheck or TweetsToRSS apps not starting in Mate desktop?
Add the notification area (system tray) applet to an existing panel (desktop toolbar).