Total noob here

ompiling .pio\build\STM32F103RC_btt_512K\src\src\HAL\STM32F1\eeprom_bl24cxx.cpp.o
Marlin\src\HAL\STM32F1\HAL.cpp:33:10: fatal error: STM32ADC.h: No such file or directory

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#include <STM32ADC.h>
compilation terminated.
Compiling .pio\build\STM32F103RC_btt_512K\src\src\HAL\STM32F1\eeprom_flash.cpp.o
*** [.pio\build\STM32F103RC_btt_512K\src\src\HAL\STM32F1\HAL.cpp.o] Error 1
In file included from d:\marlin-2.0.x\marlin\src\inc\marlinconfig.h:49:0,
from Marlin\src\HAL\STM32F1\HAL_SPI.cpp:32:
d:\marlin-2.0.x\marlin\src\inc\SanityCheck.h:2434:23: error: ‘X_SLAVE_ADDRESS’ was not declared in this scope

So, where and when did you download Marlin, and what configuration files and platformio.ini are you using?

today 19 feb
downloaded from here:
git clone GitHub - MarlinFirmware/Marlin: Marlin is an optimized firmware for RepRap 3D printers based on the Arduino platform. | Many commercial 3D printers come with Marlin installed. Check with your vendor if you need source code for your specific machine.
git clone GitHub - MarlinFirmware/Configurations: Configurations for Marlin Firmware

got the configuration for an ender 3 pro from the configurations, adapted it for the Bigtreetech SKR mini E v1.2, even adapted the pins bug in pins_BTT_SKR_MINI_E3.h
And changed the ‘env_default’ to STM32F103RC_btt_512K as instructed.
I am willing to believe that somehow this was my fault but the guy that wrote the instructions has a working configuration like this and many other ender 3 pro owners are satisfied with the explanation and frankly so am I