Noob - Fern Tree Not Working Properly - Occlusion Error


I've placed some fern trees around my scene, and when I play, I get an error saying:

The tree Fern must use the Nature/Soft Occlusion shader. Otherwise billboarding/lighting will not work correctly.

What does this mean? I can't find the Nature/Soft Occlusion in my assets and what do I apply it to?

Nature/Soft Occlusion is a shader. you need to change the shader of the leaves or the trunk of the tree to Nature/soft...

Please show a picture of choosing the shaders?