The jagged, hard appearance of the shadows is caused by insufficient lightmap resolution. Increase the lightmap resolution in the settings for your static meshes.
The dark patches near corners are caused by a combination of the previous problem and poor lightmap UVs. Making lightmap UVs is difficult and requires practice and experience. You can take a look at some documentation on how to do it better here: Unwrapping UVs for Lightmaps | Unreal Engine Documentation