Creating a fulfillment "Line items are already fulfilled" but there are no fulfillments on the order

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app name: SellingExpress
When we used the reset api url 'POST /admin/api/2020-10/orders/{order_id}/fulfillments.json ' to create a new fulfillment
,it will return the error 'Line items are already fulfilled'
But the order was not fulfilled,
we had check the
fulfillments from order details through the shopify background or used the reset api url 'GET /admin/api/2020-10/orders/{order_id}/fulfillments.json ' to Retrieves fulfillments associated with an order ,and find the order was no fulfilled.
But there are a few successes per hour,No more than 10,Is it caused by the listing status of our app?
Below is the JSON code for create a new shipment:
{"fulfillment": {"location_id":"32977190946","notify_customer":"false","tracking_company":"USPS","tracking_number":"SUA201019BIMJ1000030"}}

I want to know what is the problem that causes, is there a call limit on our app?