I missed Eternal Chains event. So I can never make Kyouka Uzen's costume +5?

The second part of the event gives me Nei's currency. The costume mission says I need to spend 7500 demon force flag. Even though the shop is open, There is no way for me to earn demon force flags anymore so it stays 0/7500 in the mission tab. I wish nei's currenct would progress that mission too