Hi @PaulYang - thanks for reaching out. I was able to confirm that the fulfillment ID you provided appears to be a valid one, so it is odd that it’s not returning the expected response (per the documentation that you shared).
Could you share a Request ID ( also called a Shopify X-Request-ID) that would have be returned in the response headers to your API call? This would allow us to pull more in depth logs on our end so that we can see what may have caused the 404 response.
Hope to hear from you soon - let us know if we can clarify anything on our end.