Configuring of MKS_GEN_L V1.0
The motherboard definition in configuration.h should be written like:

#define MOTHERBOARD BOARD_MKS_GEN_L


actually the word MOTHERBOARD was missing and there is no need to write  53 //MKS GEN L at the end.

The number definition is declared in boards.h - you just confused the two files, indeed you should not forget to define the motherboard constant itself. Do note that this is basically a RAMPS board, see pins_MKS_GEN_L.h.