Hey! Thank you for playing the game and I'm glad I was able to make something you enjoy :)
"train Darkness in the sexual art of torture and roleplay" requirement refers to the status of your romantic relationship with Darkness. In the event prior to "Prisoner of War", you would've seen a choice with a heart (romantic) and a broken heart (ignore her), and I'm afraid if the requirement is not fulfilled you might have chosen the latter, which locks you out of all the romantic scenes with her.
You could use a cheat code to change that, although that might result in some unintended consequences and narrative inconsistencies, since you're already quite far in the story