Problem with a dark cloud when i'm Placing Trees

Hello, my name is David and i have a problem with my unity project. When I try to place trees on the terrain, a dark cloud appears and it looks very bad. If I try to place a forest, the entire sky comes dark because of the cloud. Is the first time I have t$$anonymous$$s problem, could you help me? Thanks :)

p.d- The shadow looks like the silhouette of the trees, but bigger