A static mesh that is used in a scene in a specific way may not need all it’s faces, as they would only be there to cost performance without being visible to the viewer. Using meshes like this is cost efficient, but may cause problems with lighting. as they do not have backfaces to cast shadows from.

There are two ways to fix this:

Set Static Mesh Actor as two sided

Set material as two sided

A more costly option is to set the material for the mesh as two sided, as it will render the material to the other side of the face aswell. This might be a good option if you are not sure about the visibility of a backface, but still want to use a mesh without one.