Sorry, for context (and apologize if this is the wrong room) is WSDL SuiteTalk 2019_2 API.

NS REST Webservice is still not feature complete and has many things - POs included - listed as BETA. So what you're trying to do may not be available yet (I haven't used the REST Webservices much so can't say 100%). If that's the case, you may have to use the SOAP Webservice or write your own RESTlet. REST Browser for reference: https://system.netsuite.com/help/helpcenter/en_US/APIs/REST_API_Browser/record/v1/2020.2/index.html