I have both sell/fulfillment & sell/fulfillment.readyonly scopes enabled against my developer account and have the required OAuth user token minted to link with the seller account.
I have successfully retrieved the order details via the Trading API but passing either the legacy OrderId or the new ExtendedOrderId into the getOrder method of the fulfilment API returns with an error code of 32100 and a manreadable of "Invalid Order Id".
I'm happy the OAuth token is valid as replacing it with garbage tells me specifically the token is wrong. I'm happy the OrderId I'm passing is correct also as I have confirmed this with the seller. The buyer has fully completed the checkout process also.
What am I doing wrong?