You have to check "Terrain lighting" in the (advanced) map compiler options:
Terrain lighting A7.76
Generates lightmaps not only for blocks, but also for terrains. Terrain lightmaps are cast by the level geometry and by the terrain itself.
As models normally do not cast shadows, this only affects level lighting and blocks. The terrain normally receives shadows by (dynamic) models if there SHADOW flag is set, what type of shadows do you use (stencil, decal, ...)?
Btw. the next A8 version will support static shadows from static models, too:
WED & MED Improvements
... Static shadows generated from models 100%