Read Only message when attempting to SAVE a new PurchaseOrder or PurchaseInvoice

I need to know if  the SDK will allow you to SAVE a PurchaseOrder or PurchaseInvoice

So far no success.

Getting error that the entity is READ-ONLY.

Also, I do not see a "CREATE" method for purchase order lines or purchase invoice lines.

I need to know if this is designed that way or if I am doing something wrong or I am missing something.

Please reply.