As Althaen said, you can’t directly access BrakeInput as it is a protected variable. However if you look in WheeledVehicleMovementComponent.h there is a function called CalcBrakeInput() that is effectively a getter for the BrakeInput variable. It’s also public so it can be called from outside the class.
API: UWheeledVehicleMovementComponent::CalcBrakeInput | Unreal Engine Documentation