Namespace can only contain types and namespace declarations. Please help me

You're getting that error because you are trying to declare the "GameObject targetObject" variable in the namespace. This has to be done in the class body, so you need to move it down like this:

 using UnityEngine; 
 using System.Collections; 
 
 public class CameraFollow : MonoBehaviour {

          public GameObject targetObject;   //<-- MOVE TO HERE
          // Use this for initialization
          void Start () {
          
          }
          
          // Update is called once per frame
          void Update () {
          float targetObjectX = targetObject.transform.position.x; 
          Vector3 newCameraPosition = transform.position;
          newCameraPosition.x = targetObjectX;
          transform.position = newCameraPosition;
          }
      }