Ok thanks I will try to check this out. It act like if someone or something would press the motor cover plate under the platform but there is nothing. So probably the switches

If you lift up the platform using the ground controls, there is a cover that covers the drive motors under the operator platform. If this cover is depressed it will give off this alarm as it thinks there is an object under the operator platform. One of the switches may be bad or there could be something on this cover setting it off.
You can take off the cover and you will see the metal support bars that move and activate the switch.