Thank you for reaching out to the Shopify community.
It appears that you are experiencing an issue with the fulfillment resource. Please ensure that you have sufficient access to use this resource. The following scopes are required to create a fulfillment for the order:
Once you have confirmed that you have all the necessary access, please follow the instructions outlined below:
Make an API call named "Fetch order fulfillments" and pass the order id to it.
Once you receive a response from the fetch API, utilize the id from the fulfillment_orders array in the next API call named "Create order fulfillment".
Here is the Postman collection for further reference/additional use.
If you encounter any difficulties or require further assistance, please do not hesitate to let us know. We are here to support you and ensure a smooth resolution of the issue.