I use the following code to create Parallax Scrolling: http://www.david-gouveia.com/portfolio/2d-camera-with-parallax-scrolling-in-xna/

Parallax Scrolling is working but I don't know how to focus the camera on the player. If the player moves, then the camera doesn't follow the player. The player leaves the screen when I'm moving it.

I use the following code(as described in the tutorial), but it's not working:

 // Updates my camera to lock on the character
 _camera.LookAt(player.Playerposition);

What can I do so that the player is always in the center of the screen/camera?

My player class:

public class Player
{
    Texture2D Playertex;
    public Vector2 Playerposition = new Vector2(400, 240);
    private Game1 game1;

    public Player(Game1 game)
    {
        game1 = game;
    }

    public void Load(ContentManager content)
    {
        Playertex = content.Load<Texture2D>("8bitmario");
        TouchPanel.EnabledGestures = GestureType.HorizontalDrag;
    }

    public void Update(GameTime gameTime)
    {
        while (TouchPanel.IsGestureAvailable)
        {
            GestureSample gs = TouchPanel.ReadGesture();
            switch (gs.GestureType)
            {
                case GestureType.HorizontalDrag:
                   Playerposition.X += 3f;                     
                break;
            }
        }
    }

    public void Render(SpriteBatch batch)
    {
        batch.Draw(Playertex, new Vector2(Playerposition.X - Playertex.Width / 2, Playerposition.Y - Playertex.Height / 2), Color.White);
    }
}

In Game1, I update the player and camera class:

    protected override void Update(GameTime gameTime)
    {

        // Updates my character's position
        player.Update(gameTime);

        // Updates my camera to lock on the character
        _camera.LookAt(player.Playerposition);

        base.Update(gameTime);
    }

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);

        foreach (Layer layer in _layers)
          layer.Draw(spriteBatch);

        spriteBatch.Begin(SpriteSortMode.Deferred, null, null, null, null, null, _camera.GetViewMatrix(new Vector2(0.0f, 0.0f)));
          player.Render(spriteBatch);
        spriteBatch.End();

        base.Draw(gameTime);
    }