Issues With Motor Groups

should be

motor_group leftMotors = motor_group(leftWheelMotorA, leftWheelMotorB);

In addition, take the

motor leftWheelMotorA = motor(PORT1, ratio18_1, true);
motor leftWheelMotorB = motor(PORT2, ratio18_1, true);

should not be inside the

void vexcodeInit (void) {
//nothing to initialize
}

section on robot-config.cpp.
Hope that helps!

This post on VEXcode V5 Text Version 1.0.1 Release should help as well: