Error 'Resource not found for segment 'A_PurchaseOrderItemType'' happens when using patch option via odata service API API_PURCHASEORDER_PROCESS_SRV to change a PO.
SAP S/4HANA Cloud All Versions
Patch option can only be used for existing objects in system.
Try to find out correct object in system and patch again, or use Post option to create an object basing on business process.
KBA , MM-PUR-PO , Purchase Orders , Problem