Version 3.0.4.21117
it seems, that the version 3.0.4.21117 for OSX Mojahwe doesn't work in the way it should.
Beginning with a new sheet, the program breaks down. You can open only existing files.
The next problem: after a while i can't put notes with the keyboard, only with the mouse. Closing the program and opening it again was the way to repair this bug.
Comments
See various other reports here, it's a known issues some Mac users are experiecing. So, start MsueScore by opening an existing score, then oen new ones from there. You can also disable the Start Center in Edit / Preferences and then start normally after that.
A for issues with your keyboard, if you encounter this again, please start a new thread and attach your score and steps to reproduce the problem.
see #285057: MuseScore 3.0.4 on macOS is crashing on startup.