Hi Rhidor,
It looks like the Landscape Grass Type does not support static lighting, at least not at the moment. This is the warning being thrown because of this.
This tool was originally designed for larger open worlds where you would really use Dynamic lighting over any static lighting. So this may just be a limitation of the implementation at the moment. I entered UE-20766 to address this.
As a workaround for the time being, it would probably be better to use the Foliage Tool to paint your objects to the Landscape until this has been resolved. However, there is no time frame for when this will be addressed, so it could be a little while.
Using Force no precomputed lighting will only help if you don’t use any static lighting. That is intended for only using fully Dynamic Lighting in your scene.
I hope this helps at least explain why you’re seeing this message.
Tim