OS X Catalina and later require that you give permission to Bria for Screen Recording capabilities. Catalina should ask you to grant Screen Recording permission when first starting ScreenShare.
If you experience unexpected issues when using ScreenShare, check whether you have granted the permission to the app in OS System Preferences -> Security & Privacy -> Screen Recording.
Note that the app needs to be restarted once you grant permissions.
You can also try toggling it off and on again if permissions is already granted but restarting the app doesn't help.