Here's why:
* Sectumsempra is a spell, not a potion: The curse Sectumsempra is a powerful spell created by Severus Snape. It's not a potion that can be brewed.
* No potion brewing in the game: The DS game doesn't feature a potion brewing system. You primarily use existing potions you find or purchase.
What you *can* do:
* Learn the Sectumsempra spell: Later in the game, you'll be able to learn Sectumsempra as a spell from a certain character. It's a powerful but dangerous spell.
* Use existing potions: The game focuses on using existing potions like Wiggenweld (healing), Felix Felicis (luck), and others for different scenarios.
Let me know if you have any other questions about the game!