Problem in Product Modal of Inventory Management System

Thanks Brother Rizwan, I’m watching the series of Inventory Management System from your Youtube Chanel & learning a lot of things. In part-15 (Title of video: Inventory Management System | Adding Products | PHP, Mysqli, Ajax | Part 15; on Dec 29, 2017)
Until 9:37 I have completed a product table in phpmyadmin, working with product.php,
main.js but what is wrong with me, I don’t understand as the category is not fetching into the product modal
with the following codes & what should I do to go forward?  please help. Without solving this, I can't go forward with this series.

main.js:

// Fetch Category
	fetch_category();
	function fetch_category() {
		$.ajax({
			url : DOMAIN+"/includes/process.php",
			method : "POST",
			data : {getCategory:1},
			success: function(data) {
				var root = "<option value='0'>Root</option>";
				var Choose = "<option value=''>Choose Category</option>";
				$("#parent_cat").html(root+data);
				$("#select_cat").html(choose+data);
			}
		})
	}


 product.php:
<!-- Modal -->
<div class="modal fade" id="form_products" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">Add New Product</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        <form id="product_form" onsubmit="return false">
          <div class="form-row">
            <div class="form-group col-md-6">
              <label>Date</label>
              <input type="text" class="form-control" name="added_date" id="added_date" value="<?php echo date("Y-m-d"); ?>" readonly>
            </div>
            <div class="form-group col-md-6">
              <label>Product Name</label>
              <input type="text" class="form-control" name="product_name" id="product_name" placeholder="Enter Product Name" required>
            </div>
          </div>
          <div class="form-group">
            <label>Category</label>
            <select class="form-control" id="select_cat" name="select_cat" required>
              
            </select>
          </div>
          <div class="form-group">
            <label>Brand</label>
            <select class="form-control" id="select_brand" name="select_brand" required>
              
            </select>
          </div>
          <div class="form-group">
            <label>Product Price</label>
            <input type="text" class="form-control" id="product_price" name="product_price" placeholder="Enter Product Price">
          </div>
          <div class="form-group">
            <label>Quantity</label>
            <input type="text" class="form-control" id="product_qty" name="product_qty" placeholder="Enter Quantity">
          </div>
          <button type="submit" class="btn btn-success">Add Product</button>
        </form>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-danger" data-dismiss="modal">Close</button>
      </div>
    </div>
  </div>
</div>