New version

First, try to run the program by double clicking an existing score then disable the start center if it runs

next, try to revert to factory settings from the command line https://musescore.org/en/handbook/revert-factory-settings

I have also just updated to MuseScore 3.5, but it QUITS whenever the Start Centre opens! The app is installed on MacMini (late 2014) OS Catalina V 10.15.6, which has just had an update which could be the problem.
However, I found that if I selected a MuseScore file and opened the file from Open With > MuseScore 3.5.app, then 3.5 opens and runs OK since this action avoids the Start Centre routine. Once 3.5 was running, I clicked on Preferences, General, and UNTICKED the box for Show Start Centre, then clicked Apply and OK.
3.5 can now be opened and runs,....... but will still quit if Start Centre is selected!