ninja: error: rebuilding 'build.ninja': subcommand failed

[aledoors]

I get the following error when I try to build the program into the microcontroller. Does anyone know what I'm doing wrong?

[2/6] cmd.exe /C "cd /D D:\Eclipse\Trabajo\ESP32_oled_i2c_u8g2\build\bootloader && C:\Users\Alejandro.espressif\tools\cmake\3.13.4\bin\cmake.exe --build ."
ninja: no work to do.
[3/4] cmd.exe /C "cd . && C:\Users\Alejandro.espressif\tools\xtensa-esp32-elf\esp-2020r2-8.2.0\xtensa-esp32-elf\bin\xtensa-esp32-elf-g++.exe -mlongcalls -Wno-frame-address @CMakeFiles\app-template.elf.rsp -o app-template.elf && cd ."
FAILED: app-template.elf
cmd.exe /C "cd . && C:\Users\Alejandro.espressif\tools\xtensa-esp32-elf\esp-2020r2-8.2.0\xtensa-esp32-elf\bin\xtensa-esp32-elf-g++.exe -mlongcalls -Wno-frame-address @CMakeFiles\app-template.elf.rsp -o app-template.elf && cd ."
c:/users/alejandro/.espressif/tools/xtensa-esp32-elf/esp-2020r2-8.2.0/xtensa-esp32-elf/bin/../lib/gcc/xtensa-esp32-elf/8.2.0/../../../../xtensa-esp32-elf/bin/ld.exe: esp-idf/esp32/libesp32.a(cpu_start.c.obj):(.literal.main_task+0x10): undefined reference to app_main' c:/users/alejandro/.espressif/tools/xtensa-esp32-elf/esp-2020r2-8.2.0/xtensa-esp32-elf/bin/../lib/gcc/xtensa-esp32-elf/8.2.0/../../../../xtensa-esp32-elf/bin/ld.exe: esp-idf/esp32/libesp32.a(cpu_start.c.obj): in function main_task':
C:/esp-idf/components/esp32/cpu_start.c:544: undefined reference to `app_main'
collect2.exe: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
Build complete (3 errors, 0 warnings): D:\Eclipse\Trabajo\ESP32_oled_i2c_u8g2\build

Thanks in advance