Your return input seems to include an Order LineItem, but the “fulfillmentLineItemId” field requires a FulfillmentLineItem. This is specified in the docs:
The ID of the fulfillment line item to be returned. Specifically, this field expects a FulfillmentLineItem.id.
Hey all - thanks for getting in touch - and for waiting on this response. I can’t say for sure why your calls aren’t working on my end without approval from the merchant to investigate further (or if you were to get in touch with us through our Partner Support team we can authenticate that way), but broadly speaking, you’ll want to make sure you’re using the fulfillment line item object here and not the fulfillment order line item object.
To clarify, a fulfillment order is created automatically for inventory items that are associated with a particular fulfillment location/fulfillment service, and a fulfillment is what the location/service has control over - which is why you’d want to use the fulfillment line item object in this case.
I’d just make sure to confirm that you’re referencing the right type of line item as well as confirm if the ID number for the item is correct. This should resolve the issue, but let me know if you encounter any further errors - happy to keep digging into this.