Y-min stay triggered
This MKS-GEN-L board is in essence a plain RAMPS board, and as such it uses the same pinout apart from some specific pins. Do note the include of the RAMPS pinout in the MKS-GEN-L at the end of the file. 

From your comment you have changed the pins 14 and 15, in doing so, you need to plug the Y-min endstop in the Y-max socket after uploading the modified firmware. With the unmodified firmware (no pin switch), if the Y-endstop works in the Y-max socket, this means that you have a hardware issue. The pin labeled 14 is connected to the Y-max instead of the Y-min. If you did the pinout switch and left the endstop in the Y-min socket, then you can conclude that the 15 pin is not connected to the Y-min socket. Basically you don't have a 15 pin that is exposed. Could be that there is an issue on the board with the traces or the soldering.