Help me...

using UnityEngine;
using System.Collections;

public class CrowdAI : MonoBehaviour {

public float sensorLenght = 5.0f;
public float speed = 10.0f;
public float directionValue = 1.0f;
public float turnValue = 0.0f;
public float turnSpeed = 5.0f;
Collider myCollider;
// Use this for initialization
void Start () {
	myCollider = transform.GetComponent<Collider> ();
}

// Update is called once per frame
void Update () {
	RaycastHit hit;
	int flag = 0;
	//right sensor
	if (Physics.Raycast(transform.position * transform.right, out hit, (sensorLenght + transform.localScale.x))){
		if (hit.collider.tag "Obstacle" || WheelHit.collider == myCollider ) // error = 4
		{
			return;
		}

		turnValue -=1;
		flag++;
	}
	//left sensor
	if (Physics.Raycast(transform.position * -transform.right, out hit, (sensorLenght + transform.localScale.x))){
		if (hit.collider.tag "Obstacle" || WheelHit.collider == myCollider ) // error = 4
		{
			return;
		}

		turnValue +=1;
		flag++;
	}
	//front sensor
	if (Physics.Raycast(transform.position * transform.forward, out hit, (sensorLenght + transform.localScale.z))){
		if (hit.collider.tag "Obstacle" || WheelHit.collider == myCollider ) // error = 4
		{
			return;
		}
	
		if (directionValue == 1.0f) {
			directionValue -= 1;
		}
		flag++;
	}
	//back sensor
	if (Physics.Raycast(transform.position * -transform.right, out hit, (sensorLenght + transform.localScale.z))){
		if (hit.collider.tag "Obstacle" || WheelHit.collider == myCollider ) // error = 4
		{
			return;
		}
	
		if (directionValue == -1.0f) {
			directionValue = 1;
		}
		flag++;
	}

	if (flag == 0){
		turnValue = 0;
	}
		
	transform.Rotate(Vector3.up * (turnSpeed * turnValue) * Time.deltaTime);

	transform.position += transform.forward * (speed*directionValue) * Time.deltaTime;
}

void OnDrawGizmos() {
	Gizmos.DrawRay (transform.position, transform.forward * (sensorLenght + transform.localScale.z));
	Gizmos.DrawRay (transform.position, -transform.forward * (sensorLenght + transform.localScale.z));
	Gizmos.DrawRay (transform.position, transform.right * (sensorLenght + transform.localScale.x));
	Gizmos.DrawRay (transform.position, -transform.right * (sensorLenght + transform.localScale.x));
}

}

By looking at it your code definitely misses a == in these lines:

if (hit.collider.tag "Obstacle" || WheelHit.collider == myCollider )

solution:

if (hit.collider.tag == "Obstacle" || WheelHit.collider == myCollider )