Check the \esp-idf\tools\cmake\project.cmake file, it define the component directory of CMakeLists.txt.
if(EXISTS "${CMAKE_CURRENT_LIST_DIR}/main")
__project_component_dir("${CMAKE_CURRENT_LIST_DIR}/main")
endif()

So if the app_main function directory is not this main directory, need to modify ...

Check the CMakeLists.txt file of the source code directory ,add it into compile.

# This file was automatically generated for projects
# without default 'CMakeLists.txt' file.
#FILE(GLOB_RECURSE app_sources ${CMAKE_SOURCE_DIR}/src/*.*)
#idf_component_register(SRCS ${app_sources})
idf_component ...