I believe the problem is that you won’t have any effect on things if you do your comp.Start() from the constructor - RobotDemo(). It is much more normal to see the starting of the compressor at the beginning of OperatorControl() outside of the while loop. And if you need the compressor in Autonomous, you would also call comp.Start() in the beginning of Autonomous().
Our code calls compressor start in the robot class constructor. It works fine. Compressor takes time to charge up the air tanks so you would want to start the compressor as soon as possible. If not the robot constructor, it should be at least the beginning of RobotInit.