There is a quest for the Dwarven faction which requires minimum 10 public order in your main province. This quest cannot be completed if public order is 0.
They did at some point update the trainer because at one time the skill points only worked on lords so if you re download it, it should work fine. It works for me.
There are mods in the steam workshop that cover that. Just do a search for diplomacy and you'll find em
Once you have used it in a fight. Remember to deactivate it. Then reactivate and select the troops before you start the battle. Then boom all should be groovie.