Foliage Fill Tool not working

Hi all,
I’m trying to cover my landscape with grass, rocks, trees, etc… the map is 4km x 4km and I would like to use the fill tool because the brush is too slow.

Unfortunately when I ctrl + click the terrain nothing happens, does someone know why?

also I have another question.

How can I turn a spline into a river?


Fill tool on landscapes disabled because of potential performance issues. Landscapes tend to be very big so filling all surface with foliage will be too slow.

Thank you! I understand now