Maybe in the fanfic, Sakura has committed some unforgivable betrayal. For example, she might have broken a very important promise to the protagonist or she has caused great harm to someone the protagonist cares about deeply.
It could be that Sakura has been involved in some sort of deception in the story. Perhaps she has been lying about her identity or her intentions all along, and when the truth comes out, the character in the fanfic feels that it's just too much to forgive. This kind of plot device is often used in fanfics to create drama and conflict.
Perhaps it's a rival of Sakura's. In many fanfics, there are rivalries, whether it's for power, love, or recognition. If Sakura outmaneuvered this rival in an unfair way, like cheating in a competition for a coveted position or a magical power, the rival would hold a grudge and find it hard to forgive her.
It could also be due to the general nature of Majin Boo. He was initially a villain created for evil purposes. His appearance and the way he carried out his actions were often grotesque and malevolent. Bulma, who is used to a more civilized and scientific environment, might be repulsed by his very existence and find it unforgivable that he was able to cause so much trouble in their world.
Well, without reading the specific 'I can't forgive you Sakura fanfic', it's hard to say exactly. But usually, in a fanfic with such a title, it might be about a conflict between the characters where one character named Sakura has done something that the other can't overlook. Maybe she betrayed a friend or made a decision that had negative consequences for others.
Another possibility is jealousy. Maybe in the fanfic, Sakura shows more interest in another character, like Sasuke perhaps, and Naruto, who has his own feelings for Sakura, snaps out of envy. It's a common trope in fan - made stories to play with the characters' emotions like this.
One reason could be that she realizes she has been too dismissive of Naruto's feelings for her. She might have led him on or been cruel in rejecting his advances in the past. Another reason is that she finally understands how much Naruto has sacrificed for the village and for her personally, and she feels guilty for not showing him more respect and gratitude earlier.
It could also be due to Sakura constantly doubting Naruto's abilities in the story. Naruto has always been looked down upon by many in the original series, and if in the fanfiction Sakura continuously expresses her lack of faith in him, it could lead to his unforgiveness. For example, she might have refused to believe in Naruto's new jutsu or his plans to save the village, which made Naruto feel really disappointed and unable to forgive her.
In the fanfic, it could be that Sasuke doesn't appreciate Sakura enough. She might pour out her love and support for him, but he doesn't return it in the way she hoped. Sakura may have expected a more reciprocal relationship where her feelings are acknowledged and valued. Since that's not the case, she starts to regret her choice of Sasuke.
One possible reason could be that Sakura has grown and changed a great deal. Maybe she has become stronger both physically and mentally, and this newfound strength and confidence are what draw Sasuke to her. For example, if she has made significant contributions to the village or has mastered some new and powerful jutsu, Sasuke would start to see her in a new light.
It could be because of a dark power corrupting Sasuke. Maybe an ancient evil force takes over him and makes him see Sakura as an enemy. In some fanfics, this kind of external influence can completely change a character's actions. Also, it might be related to Sasuke's pursuit of power. If he believes Sakura is standing in the way of his ultimate power - seeking goal, he could be driven to kill her.
One possible reason could be Sasuke's focus on his revenge. His thirst for vengeance against Itachi might make him blind to Sakura's feelings and neglect her as he is constantly preoccupied with getting stronger to achieve his goal of revenge.