Did a search and found that 'drag and drop' can be disabled, learned something new. Here's the solution to re-enable it:Thanks for all your replies.
DarkRob's "Z" solution worked, and with it, I seem to have stopped having trouble with Le Cent Jours. I still can't "drag and drop." If I understand what you've told me, all I should do is start the game, go to Modifications,left click on the mod I want to move, left click, and drag up or down. Is that correct?
"When drag and drop doesn't work, left click a file in Windows Explorer or File Explorer, and keep the left click mouse button pressed. While the left click button is held down, press the Escape key on your keyboard, once. Release the left click mouse button. Try to drag and drop again. This feature should work now." Jul 24 2019