That prize is the 3rd day prize, which means, you will only receive the Event Defense Scroll when you complete 3 consecutive days. At the moment it says you only have 2 days.
So tomorrow if you log in, you will have 3 consecutive days, and you should be able to claim the 3rd day prize.
im pretty sure i logged in the game and event page every single day and one day it was reset. there is certainly a bug.