Trouble installing on a Mac with Catalina

I'm using Catalina and I just installed a new application (Musescore 3) but I can't launch it. According to installation troubleshooting instructions:

Note: As of macOS 10.15 "Catalina", the system has to be told to permit MuseScore to be allowed to access user parts of the file system. The first time you use it on Catalina, it will ask you if you wish to permit it; of course, answer "yes", but if you bypass this by accident, you can set it via System Preferences > Security and Privacy > Privacy > Files and Folders. Unlock with your Admin credentials, then navigate to MuseScore (whichever version(s) you have and want), add it or them to the list of apps, and select "Documents" and "Downloads" folders, or as you prefer.

However, inside Security & Privacy/Files and Folder the + sign is faded and I don't have the option to add any new application. How can I add the new application to the list?