I had the same problem as the OP. Jupyter Server would not start after install of the .app.dmg package.
I can report success launching Sage from the Applications icon after the following steps: 1) Grant Full Disk Access in System Preferences->Security->Privacy (as suggested previously) 2) Launch Sage from the command line in the executable's installed directory. In a terminal window: cd /Applications/SageMath-9.0.app ./sage -n 3) The executable should "patch" or rewrite all of the required paths to match their actual locations in your particular Catalina install. After that, it should open a Jupyter tab in Safari. 4) When you see the Jupyter tab in Safari: click the Quit button. This will stop the Jupyter Server, but that's ok. 5) Launch Sage using Applications->SageMath-9.0. All should be well at this point, but your actual mileage may vary. Hope this helps. -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-support/354982d2-b75d-4424-a60a-fd749725d42f%40googlegroups.com.