Fulfillment issue with Third Party Fulfillment service app

Shopify Discussion
1

I have a public app that creates a fulfillment service and the configuration to create one is given below (this is PHP/Laravel code)

json_encode([
   "fulfillment_service" => [
        "name" => config('custom.fulfillment_service_name'),
        "callback_url" => route('shopify_callBack'),
        "inventory_management" => true,
        "tracking_support" => true,
        "fulfillment_orders_opt_in" => true,
        "requires_shipping_method" => true,
        "format" => "json"
   ]
]);

After I have created the fulfillment service I am trying to call the fulfillment API (Version 2022-01)

'orders/'.$shopify_order->id.'/fulfillments.json'

It returns the response like this

{"httpCode":422,"sBody":"{\"errors\":{\"base\":[\"The fulfillment service has registered for fulfillment order based fulfillments.\"]}}"}

Note: I did go through this document
https://shopify.dev/apps/fulfillment/fulfillment-service-apps#requirements

and I created the <callback_url>/fulfillment_order_notification but I did not receive any request on this endpoint.

Please help here. Thanks in advance!