I realize it’s been a while since the question, but when an error of “* to incomplete class type is not allowed” is usually an issue with the file you’re working in (most likely the .cpp if you’re implementing) doesn’t have an include line for the header file where the type is defined. So for this specific issue, you would want to add to the include section of your file
#include "Runtime/Online/HTTP/Public/HttpModule.h"