For me Unreal 4.26 incorrectly generated the include for MainMenu.h it added the sub folder instead of leaving it relative which causes the build to fail as it can’t find MainMenu.h
fatal error C1083: Cannot open include file: 'MenuSystem/MainMenu.h': No such file or directory
If this happens to you open MainMenu.cpp and change the include to
#include "MainMenu.h"
Also if you just build in visual studio it didn’t add the folder structure and class to the unreal GUI I did the compile again from the Unreal UI and everything updated.