Just to verify...did you change the Config.h file to indicate the MKS Gen L. See code line below:
#define PINMAP MksGenL2 // OFF, Choose from: MiniPCB, MiniPCB2, MaxPCB, MaxPCB2, STM32Blue, <-Req'd
// Ramps14, MaxESP2, MaxESP3. Check Constants.h for more info.
On my MKS, I have Serial A, B and C all set to 9600 baud. See below
#define SERIAL_A_BAUD_DEFAULT 9600 // 9600, n. Where n=9600,19200,57600,115200 (common baud rates.) Infreq
#define SERIAL_B_BAUD_DEFAULT 9600 // 9600, n. See (src/HAL/) for your MCU Serial port # etc. Option
#define SERIAL_B_ESP_FLASHING OFF // OFF, ON Upload ESP8266 WiFi firmware through SERIAL_B with :ESPFLASH# cmd. Option
#define SERIAL_C_BAUD_DEFAULT 9600 // OFF, n, ON for ESP32 Bluetooth. Option
#define SERIAL_C_BLUETOOTH_NAME "OnStep" // "On..", Bluetooth device name for ESP32. Option
