Okay so i was playing around in the Engine and ive made a forrest with some houses n stuff. but now i wanna bake the Lightning for it.
So i tried the “Preview” quality, but after a few hours it was stil 0%.
So i tried to remove the grass and then it took like 2 minutes.
So is it possible to make the grass not cast shadows or whatevers taking so much time?
I get this message
Thanks in advance! 
Basically, is not recommended do lightmaps to the foliage, because the build time increases significantly. You should disable cast static shadows in the foliage parameter for more performance. However, the error that are you getting in the screenshot, is because the fbx uvs they are not set correctly.
you can choose cast or not, the shadows, just must go to the foliage editor. Select grass and then below, find the checkbox that say Cast Static Shadow, and disable it.
Cheers!
Correct. It is not recommended to use baked lighting for large foliage scenes like the one you have linked. There are two reasons for this. One, because the lighting cache that needs to be calculated takes a lengthy time to calculate as well as decreasing FPS when drawn after baking. Using Dynamic lights will help with both these problems. Two, there is currently a bug with baked lighting and foliage. This bug involve the lighting cache deriving from one light source. What this causes is foliage that should be in the shadow to be lit as if it were lit by your light.