When querying /purchase-orders by status, NetSuite only supports searching by specific values that are not the status values seen in the Response Body. For instance, to search for a closed purchase order use status = '_purchaseOrderClosed'.

See the full list of permissible transaction statuses for queries here: http://www.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2016_2/schema/enum/transactionstatus.html?mode=package