Victory and Glory: Napoleon is a game of grand strategy and fast-play tactical battles where you take the role of Napoleon Bonaparte and attempt to dominate the entire continent of Europe.
Checking this, I see that there is an error in the game's database. You are supposed to see a message telling you that you don't have any light cavalry to upgrade. Therefore you cannot play the card.
That <error> is shown because a text key is not properly identified. The game cannot find the text string it is looking for.
The only work-around for now is to live with this particular error. Sorry. [:(]