Create Invoice without PO in isupplier portal

Hello

I have created an invoice without PO in isupplier portal. I submited it to the buying company. The invoice is now in the AP modukle of the buying company and the status is: Never Validated and the Approval is: Rejected. The document type is not Standard invoice but Standard Invoice Request.

Why this type of invoice (Standard Invoice Request) can't be validated in AP module? Do I need to set up something else?

Do I have to set up the "Payables Invoice Approval" workflow for this type of invoices in the buying company in order to validate the invoice?