Some of the early guide quests will only complete if the hero does them on Normal difficulty because, when the game first came out, heroes wouldn’t / couldn’t have progressed into Hell I difficulty without having done the storyline stuff. These days it’s entirely possible for heroes to out-level Normal difficulty, so may be doing the storyline quest in Hell I (or higher). So, what level is your hero, and what difficulty were you doing the dungeon on?
Could you add a screenshot of your hero having killed the end boss of the dungeon, and showing the quest?
Here’s how…