Some versions require you to have a certain amount of "Intelligence" stats or specific items (like a notebook) before the final writing option appears on your PC.
World of Sisters [v0.44] By Sexy Goddess Game Studio - Itch.io World of Sisters [v0.20.1]
If you are stuck on a specific character's sub-quest (like helping a sister with her homework), ensure you have triggered the dialogue with them first before trying to use the computer. Some versions require you to have a certain
Return to your room at home in the evening or night. Interact with your computer and select the option to "Write Paper" or "Complete Assignment." World of Sisters [v0.20.1]
Interact with the bookshelves or NPCs to "study" or "collect data." This often updates your quest log to the next phase.