I did this and I would complete the focus, however nothing would appear, Even on the text of the focus it just says gain , and thats it.
The issue may be due to your idea.
The name "MON= Retrained_Cavalry" isn't valid. Try "Retrained Cavalry".
And the reward cannot be itself: "add_ideas= MON= Retrained_Cavalry"
Then, in your focus tree, you just need to change: "add_ideas= MON= Retrained_Cavalry" to "add_ideas= MON_retrained_cavalry"