My developer suddenly quit so I tried but we originally used the 2021-07 version of api and now we can't make a choice so we chose 2023-04. I've tried a number of ways and I've been given a successful response eventually, but nothing has changed in Shopify. Can you tell me how to change the code?
This is a case where you'll likely need to hire another developer to make the required changes. The endpoint that you're using (orders/order_id/fulfullments.json) has been deprecated for a couple of years now in favour of a new system called fulfillment orders.
Unfortunately this isn't a migration where we'll be able to tell you to switch a couple of params in an endpoint and get the app to work, you'll likely need help from a developer to dig into your existing codebase and make the required updates to migrate. Some core concepts have changed about how fulfillments are managed.