When I add a new subdirectory in CMake, the bug occurs

I have project that builds with CMake.
Here is project hierarchy:

Project
   Dependencies
      glfw
   Source
      CMakeList.txt
      main.cpp
   CMakeList.txt

Then I try to add GLFW to my project and my CMakeList file looks like:

cmake_minimum_required(VERSION 3.8)

# Enable Hot Reload for MSVC compilers if supported.
if (POLICY CMP0141)
cmake_policy(SET CMP0141 NEW)
set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT "$<IF:$<AND:$<C_COMPILER_ID:MSVC>,$<CXX_COMPILER_ID:MSVC>>,$<$<CONFIG:Debug,RelWithDebInfo>:EditAndContinue>,$<$<CONFIG:Debug,RelWithDebInfo>:ProgramDatabase>>")
endif()

project( Project )

# Include sub - projects.

set(GLFW_BUILD_DOCS OFF CACHE BOOL "" FORCE)
set(GLFW_BUILD_TESTS OFF CACHE BOOL "" FORCE)
set(GLFW_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)

set(MAIN "Source/main.cpp")

add_subdirectory("Dependencies/glfw")

add_executable( Project ${MAIN} )

target_link_libraries( Project glfw )

With the version of the file above, everything works fine.

When I add this line 'add_subdirectory("Source")`' to CMakeList, an error occurs in the main.cpp that the file cannot be found: 'GLFW/glfw3.h'. I want to understand why the error occurs.

main.cpp:

#include <stdio.h>
#include <stdlib.h>

#include <GLFW/glfw3.h>

int main()
{
	//glewExperimental = true; // Needed for core profile
	if (!glfwInit())
	{
		fprintf(stderr, "Failed to initialize GLFW\n");
		return -1;
	}
}

EDIT:
CMakeList.txt in Source directory:

add_executable (CMakeTarget "Game Engine.cpp" )

if (CMAKE_VERSION VERSION_GREATER 3.12)
  set_property(TARGET CMakeTarget PROPERTY CXX_STANDARD 20)
endif()

The issue is that you link Project to glfw but do not link CMakeTarget (strange name, btw) to glfw.

You need to add the line:

target_link_libraries( CMakeTarget glfw )

This is because CMake uses link libraries to determine how to set up include paths for the compiler as well as for linking. Specifically, it works because glfw has a PUBLIC target_include_directories here: https://github.com/glfw/glfw/blob/9a87635686c7fcb63ca63149c5b179b85a53a725/src/CMakeLists.txt#L140