Verify that the turn sensor on the front is positioned correctly. When the truck starts up, the front wheels will turn left then right then back to center.
Do you have the latest version of the software? The hexbug software needs to be on the latest version.
If you do the above things then you should call VEX/Hexbug support during working hours.
I’ve had three of these kits and the brains ended up replaced in two of them.
Sadly, that sensor is DEEP inside the engine area. You don’t need to take apart the entire truck but you will need to take the front apart. As I remember, taking the hood off and going that way was slightly easier than taking the bottom off.
In either direction you are looking at about 1/2 hour of disassembly/assembly time. I would take it apart, verify the the placement and then try it outside of the truck. That way if they decide you need a new sensor that you are not taking it apart AGAIN.