What’s the reason you need the transaction_id? The Transactions API is deprecated. Otherwise all refunds will have an order_id for the refund. As for the error your getting what’s the ID your using to call the Orders API. 
What’s the reason you need the transaction_id? The Transactions API is deprecated. Otherwise all refunds will have an order_id for the refund. As for the error your getting what’s the ID your using to call the Orders API.