I assume you meant to say you "can't" drive. If so, most likely cause is something is keeping either one or both of the pothole protection bars from deploying. There is a pothole protection bar on each side of the machine, those bars deploy downward when you raise the platform. If they're not fully deployed by the time the deck reaches somewhere between 8 to 9 feet it will lock out the drive function. You need to check all the linkage and components of the pothole protection bars to see if something is preventing them from full deployment. If that's not the problem, likely one of the switches that the pothole protection bars activate.