Most likely you have created your own scene, so the question is:
did you define the left and right markers? When you create a new
scene you also need to create two empty game objects and you have
to position them to mark the top left and top right corners of your
board, then in the Inspector you have to assign those gameobjects
to the Board script's members "Left mark" and "Right Mark".
Another more trivial reason is that the system cannot load the board's file either because the Full Game Execution flag is checked but you have not set the statics by code or Full Game Execution is unchecked but the board pointed in the inspector is not existent.
double check to have that step covered before troubleshooting the issue any further.