I've just discovered something interesting. I was using Matter Control version 1.7.5 but have version 2.0 on the same desktop. While attempting to connect the board, the program said that it wasn't able to because another program is already connected. Could the Arduino UI be that program? My Monoprice mini delta connects to Matter Control but doesn't have anything to do with the Arduino UI, and doesn't contain an Arduino. I wonder why version 1.7.5 doesn't show a similar error message.