After saying no you can still get the Avada Kedavra spell from Sebastian.

Meet Sebastian in Undercroft

1. Open Hogwarts map and choose The Astronomy Wing and then Defense Against The Dark Arts Classroom floo flame.

2. Go down the stairs in your right and then go to the dead end in your right to find a teleport clock.

3. Hold the enter button to teleport to the Undercroft.

https://wikigames.net/hogwarts-legacy-how-to-get-to-undercroft/

Sending Sebastian to Azkaban

If you have sent Sebastian to the Azkaban, you can no longer learn the Avada Kedavra spell from him.