After checking the tech tree, I am not able to select any units. At this point, the campaign needs a restart.
Happened to me in the middle of the game after like 5th check of my tree
Happened to me too.
To fix this, re-open the tech tree and select another civ.
Then close and you will be able to play again.
I came here to report the same issue.
In a game I couldn’t do any type of click on the screen after looking at the tech tree, although I could move the mouse around and the hotkeys on the keyboard worked perfectly. After going to the options menu I could click things again