Cannot find source file error when running CMake - c++

Question

Error from Terminal:
ics45c#ics45c-2019fall : ~/projects/proj1 $ ./build app
-- Configuring done
CMake Error at CMakeLists.txt:49 (add_executable):
Cannot find source file:
/home/ics45c/projects/proj1/app/.#main.cpp
Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
.hxx .in .txx
CMake Error: CMake can not determine linker language for target: a.out.gtest
CMake Error: Cannot determine link language for target "a.out.gtest".
CMake Error: CMake can not determine linker language for target: a.out.app
CMake Error: Cannot determine link language for target "a.out.app".
CMake Error: CMake can not determine linker language for target: a.out.exp
CMake Error: Cannot determine link language for target "a.out.exp".
-- Generating done
-- Build files have been written to: /home/ics45c/projects/proj1/out
I checked my file manager and main.cpp and #main.cpp# are in the location /home/ics45c/projects/proj1/app
Also I don't understand the other CMake errors after CMakeLists.txt either. What does it mean that it can't determine the linker language?
## Here is the CMakeLists.txt file ##
cmake_minimum_required(VERSION 3.10)
set(CMAKE_C_COMPILER clang)
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(COMPILE_FLAGS "-stdlib=libc++ -Wall -pedantic-errors -Werror -g -fstandalone-debug")
project(a.out.app)
file(GLOB APP_SRC_FILES ${CMAKE_SOURCE_DIR}/app/*.cpp)
set(APP_SRC_FILES_EXCEPT_MAIN ${APP_SRC_FILES})
list(REMOVE_ITEM APP_SRC_FILES_EXCEPT_MAIN ${CMAKE_SOURCE_DIR}/app/main.cpp)
add_executable(${PROJECT_NAME} ${APP_SRC_FILES})
set_target_properties(${PROJECT_NAME} PROPERTIES COMPILE_FLAGS ${COMPILE_FLAGS})
target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/app)
target_link_libraries(${PROJECT_NAME} pthread c++)
project(a.out.exp)
file(GLOB EXP_SRC_FILES ${CMAKE_SOURCE_DIR}/exp/*.cpp)
add_executable(${PROJECT_NAME} ${EXP_SRC_FILES} ${APP_SRC_FILES_EXCEPT_MAIN})
set_target_properties(${PROJECT_NAME} PROPERTIES COMPILE_FLAGS ${COMPILE_FLAGS})
target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/app)
target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/exp)
target_link_libraries(${PROJECT_NAME} pthread c++)
project(a.out.gtest)
file(GLOB GTEST_SRC_FILES ${CMAKE_SOURCE_DIR}/gtest/*.cpp)
add_executable(${PROJECT_NAME} ${GTEST_SRC_FILES} ${APP_SRC_FILES_EXCEPT_MAIN})
set_target_properties(${PROJECT_NAME} PROPERTIES COMPILE_FLAGS ${COMPILE_FLAGS})
target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/app)
target_include_directories(${PROJECT_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/gtest)
target_link_libraries(${PROJECT_NAME} pthread c++ gtest gtest_main)
Line 49 is the error according to the CMake error but I have no idea why:
add_executable(${PROJECT_NAME} ${GTEST_SRC_FILES} ${APP_SRC_FILES_EXCEPT_MAIN})
I just don't understand any of this, and this is also my first Stackoverflow post.