Installing MuseScore on Mac OS 10.15
I was having issues with getting MuseScore to run, so I decided to uninstall then reinstall the app. After installing, I get a warning that reads, '"MuseScore 3.3.app" can't be opened because Apple cannot check it for malicious software. The software needs to be updated. Contact the developer for more information. Safari downloaded this file today at 14:34 from cdn.jsdelivr.net' (see 
Comments
See https://musescore.org/en/faq#faq-296716
In reply to See https://musescore.org/en… by Jojo-Schmitz
Thank you. That solved it.
Now I see that extensions can't be installed. That was the issue that motivated the uninstall/reinstall.
In reply to Thank you. That solved it… by pocketwevel
See #296452: Extensions not Installing in Musescore 3.3 on macOS Catalina