This is the additional pins_FYSETC_S6_V2_0 file which has been added to the Marlin firmware. Does this help?
#include "pins_FYSETC_S6.h"
//
// EEPROM Emulation
//
#undef FLASH_EEPROM_EMULATION
#define I2C_EEPROM
#ifdef I2C_EEPROM
#undef E2END // Defined in Arduino Core STM32 to be used with EEPROM emulation. This board uses a real EEPROM.
#define E2END 0xFFF // 4KB
#endif
//
// Steppers
//
#undef X_ENABLE_PIN
#define X_ENABLE_PIN PE9
#if HAS_TMC220x
//
// TMC2208/TMC2209 stepper drivers
//
//
// Software serial
//
#undef X_SERIAL_TX_PIN
#define X_SERIAL_TX_PIN PE8
#undef Y_SERIAL_TX_PIN
#undef Y_SERIAL_RX_PIN
#define Y_SERIAL_TX_PIN PC4
#define Y_SERIAL_RX_PIN PC4
#undef Z_SERIAL_TX_PIN
#define Z_SERIAL_TX_PIN PD12
#undef E0_SERIAL_TX_PIN
#define E0_SERIAL_TX_PIN PA15
#undef E1_SERIAL_TX_PIN
#define E1_SERIAL_TX_PIN PC5
#undef E2_SERIAL_TX_PIN
#define E2_SERIAL_TX_PIN PE0
#endif
//
// Software SPI pins for TMC2130 stepper drivers
//
#define TMC_USE_SW_SPI
#if ENABLED(TMC_USE_SW_SPI)
#ifndef TMC_SW_MOSI
#define TMC_SW_MOSI PE14
#endif
#ifndef TMC_SW_MISO
#define TMC_SW_MISO PE13
#endif
#ifndef TMC_SW_SCK
#define TMC_SW_SCK PE12
#endif
#endif