Missing a mission

So i recently started trying to go for 100% of the game when I noticed one of the missions isn't loading. I have completed both zombie and plant badges. Yet for some reason the achievement "curseproof" wasn't claimed. When looking through Dave bots missions I noticed the mission I have to do to achieve the achievement is no longer available (It only shows now 5 of the 6 original missions) 

So my question is does anyone know how I can get the achievement? Or how to fix this bug?