Error message "Door push down"

I have a Beckman Allegra X-14R in my lab, which is less than 6 years old. Now, when you push the lid down, you can hear the sound of click, I guess that means the door is locked. But, it keeps showing the error message "Door push down" when to start a run. Does anyone know what the problem is, the latch assembly, the sensor or the board? Many thanks!