It might be that the high school setting in the fanfiction is too clichéd for her. Annabeth is a complex character and perhaps she feels that the typical high school drama scenarios in the fanfiction don't do justice to their story. For example, if it reduces their epic adventures to simple high school rivalries, she would likely hate it.
Another reason could be that the fanfiction doesn't respect the growth and development of their relationship. Annabeth and Percy have a deep, evolving bond. If the fanfiction simplifies it or takes it in a direction that doesn't align with how their relationship should progress, it would make her dislike it.
One possible reason could be that the fanfiction misrepresents their characters. Maybe it portrays Percy in a way that Annabeth doesn't like or goes against the essence of their relationship as she sees it.