AJAX success alert is not activating even with HTTP code: 200

Here is the checkbox that I target:

 $query = " SELECT DISTINCT(collection) FROM products";

            $featured = mysqli_query($conn,$query);
                    while ($row = mysqli_fetch_assoc($featured)){?>

                    <div class="list-group-item checkbox" style="background- 
                    color: #f4f4f4;">

                        <label><input type="checkbox" class="common_selector 
                        collection" style="" value="<?php echo 
                        $row['collection']; ?>" id='<?=$row["collection"];? 
                        >'><?php echo $row['collection'];?></label>
                   </div>
                   <?php } ?>

Here is the AJAX:

$("input:checkbox").change(function(){
            var product_collection = $(this).attr('id');    
            var action = "coll";
            console.log(product_id);


            $.ajax({
                url:"../PHP_Scripts/fetch_data.php",
                method: "POST",
                dataType: "json",
                data:{
                    product_collection:product_collection,  action:action
                },
                success:function(){
                     alert("hello");

                }
            });


      });

And here is fetch_data.php

header("Location: page_2.php");   

As you can see, I just want it to redirect to a different page. The success does not send and I can't find what is wrong with the code.

The network tab shows that the data does get sent with a code of: 200 and it does contain the data that needs to be sent. Any ideas?