It won't appear right away ;). As a rule, there's one skill that takes a long time to appear before the Special Ability (SA). I've already said what to do in this case: remove two skill slots with skills not related to the SA from the mentor, and try different variations until that skill appears. Then, remove the unnecessary skills, take the SA, and continue to develop what you need. If you want a level 24 hero to develop a super ability, it's unlikely to work. You need level 28-30.

The saying "If you keep trying, something will eventually work out!" is very appropriate here. Stock up on gold and patience :).