If the Plex app won’t start and isn’t showing any error messages, you can attempt the following procedure to see if there’s any error messages on the OS level:
Open Finder and navigate to /Applications
Select Show Content from the context menu of the Plex.app item
Navigate to Contents/MacOS
Open the Terminal app
Drag the Plex executable from the Finder window (#3) into the Terminal Window and press Return to execute it
This is no different from running the app, so it won’t succeed – but you should get a more specific error message about what’s causing it to fail starting.
Edit:
One additional question: are you running the Plex app on an Intel or M-series Mac?
It‘s a maOS beta… so there can be all kinds of issues.
I would try to completely uninstall Plex for Mac, reboot, then install it again.
Because I do that a lot for testing, I have a script I call delplex.sh
#!/bin/bash
echo ' '
rm -rf /Applications/Plex.app && echo '[1/5] deleted the app'
rm -f ~/Library/Preferences/tv.plex.desktop.plist && echo '[2/5] deleted the plist'
rm -rf ~/Library/Logs/Plex && echo '[3/5] deleted the logs'
rm -rf ~/Library/Caches/Plex && rm -rf ~/Library/Caches/tv.plex.desktop && echo '[4/5] deleted the caches'
rm -rf ~/Library/Application\ Support/Plex && echo '[5/5] deleted the application support'
echo 'Plex for Mac has been removed.'
echo ' '