Heres another CD Thread on the same issue I believe.
But to summarize, you are defining and initializing your spark max objects in your constructor, you should be defining them outside of the constructor, because otherwise they will be inaccessible outside that function. So that would look something like this
private final CANSparkMax frontLeftMotor;
private final CANSparkMax rearRightMotor;
public CANDrivetrain() {
frontLeftMotor = new CANSparkMax(0, MotorType.kBrushless);
rearRightMotor = new CANSparkMax(1, MotorType.kBrushless);
}