When you use an orderId to query a single return PO in the Return/Cancellation Request List Query API, please make sure the query parameters: createdAtFrom, createdAtTo, orderId and cancelType are present as the below sample:
https://api-gateway.coupang.com/v2/providers/openapi/apis/api/v4/vendors/C00600000/returnRequests?createdAtFrom=2022-04-01&createdAtTo=2022-04-28&orderId=26000135670000&cancelType=RETURN