The ULevelStreaming class is marked as MinimalAPI in the source code which only exposes the bare minimum of that class to be called outside that class. If you are using a source build of the engine you can edit LevelStreaming.h to change the UCLASS specifier from MInimalAPI to EngineAPI.
I have the same issue. I would like to start with that I have tried changing line 78 of LevelStreaming.h from minimalAPI to EngineAPI, and the problem still persists I have also tried changing Line 79 to say class ENGINE_API ULevelStreaming : public UObject; and removed all the other calls to ENGINE_API. I have also tried adding a ENGINE_API on line 293 and none of these have worked. The code that causes the link error is simple because I’m trying to get it to build.
I know that this is an older question but if there is a fix it would greatly help, or if there is a better way to make it so that I can randomly generate a tower with X amount of floors loading each floor independently.