This does NOT require an answer. I diagnosed my own problem, but I wanted to show everyone else the problem and the diagnosis in case you run across this.
This morning, I was editing a piece of music. I opened ScoreManager and clicked Add Instrument. The three-column instrument popup box appeared. But any click in that box caused that popup box to close. I could not select an instrument.
But I diagnosed the problem: It only occurs if I am interacting with ScoreManager on my attached HDMI monitor. If I slide the ScoreManager over to the laptop screen, it works fine. I believe the problem is related to the screen resolution of the two screens.
On my laptop with the screen resolution set at 1920x1080, the ScoreManager dialog is sized at 772x499 pixels. I can add instruments on the ScoreManager dialog.
When I slide the ScoreManager dialog onto my secondary monitor which is set to a resolution of 3824x2160, the physical size of the ScoreManager dialog jumps up to 985x624, and I cannot select new instruments.
When I slide the ScoreManager dialog onto my third screen, which is 1920x1080, the dialog pops back to 772x499, and I can once again add new instruments.