The story mission Defensive Position in Assassin's Creed Shadows is optional and will not be available if you choose the correct target in the quest The Tea Ceremony. We describe the walkthrough and rewards for the quest.
This quest will only open if you made a mistake at the tea ceremony and killed the wrong person. If you follow our guide, you won't receive it.
However, if you chose the wrong target, you will have to go to the fortress by the marker and eliminate your target. Along with Wakasa, other enemies will be waiting: you can sneak up on her unnoticed, eliminating a few sentries beforehand to kill stealthily.