WP updating post_meta associative array with Ajax

I'm cycling through an associative array and adding each child array as a row in a table.

The last column in that table is a button in which I want to use to remove that array from the parent array in the database.

Here's my markup:

<?php
$response_cost = get_post_meta( 379, '_wc_booking_pricing', true );
?>

<div id="response-cost" class="hidden"><?php echo json_encode( $response_cost); ?></div>

<?php
$i = 0;
foreach ($response_cost as $response) { ?> 
    <tr id="array-<?php echo $i; ?>">
        <!-- table construction -->
        <td><button class="remove-array" data-id="<?php echo $i; ?>"><i class="fa fa-times"></i></button></td>
    </tr>

    <?php
    $i++; 
};
?>

Here's my jQuery that takes care of removing the child array:

(function($) {
    $(document).ready (function () {
        $(function(){
            var arrString= $('#response-cost').text();
            const arr = JSON.parse(arrString);

            $('.remove-array').click(function(){
                var val = $(this).attr("data-id");
                arr.splice(val, 1);
                var arr_stringify = JSON.stringify(arr);

                $.ajax({
                    url: ajax_object.ajaxurl,
                    type: 'POST',
                    data:{
                        action: 'update_cost_rule_table_action',
                        stringified_arr: arr_stringify,
                    },
                    success: function( data ){
                        console.log( data );
                    }
                });
            });
        });
    });
}) (jQuery);

Here's the function in my function.php file:

add_action( 'wp_ajax_update_cost_rule_table_action', 'update_cost_rule_table' );
add_action( 'wp_ajax_nopriv_update_cost_rule_table_action', 'update_cost_rule_table' );
function update_cost_rule_table(){
    $response_cost = json_decode($_POST['stringified_arr']);
    update_field('_wc_booking_pricing', $response_cost, 379);
    wp_die();
}

And here's how I'm enqueuing the script:

function gt21_scripts() {
    wp_register_script( 'cost-rule', get_template_directory_uri() . '/js/cost-rule.js', array( 'jquery' ) );
    wp_enqueue_script( 'cost-rule' );
    wp_localize_script( 'cost-rule', 'ajax_object', array( 'ajaxurl' => admin_url( 'admin_ajax.php' ) ) );
};

add_action( 'wp_enqueue_scripts', 'gt21_scripts' );

I'm currently getting a 404 error for the admin-ajax.php in my console.

You write an incorrect file name.

Replace

admin_url( 'admin_ajax.php' )

With this

admin_url( 'admin-ajax.php' )