It appears something is wrong with your installation. Try Help / Revert to Factory Settings. if that doesn't help, say what OS you are on and what settings you see in Edit / Preferences / I/O.
In the future, please use the Support forum to ask for help using MuseScore. Only if a problem is confirmed to be an actual bug should you use the issue tracker.
Comments
It appears something is wrong with your installation. Try Help / Revert to Factory Settings. if that doesn't help, say what OS you are on and what settings you see in Edit / Preferences / I/O.
In the future, please use the Support forum to ask for help using MuseScore. Only if a problem is confirmed to be an actual bug should you use the issue tracker.