Only a vendor line item is allowed for net posting

What causes this issue?

You have entered a vendor posting with a document type which is set to
"net posting". This document type permits neither several vendor line
items nor a combination of vendor and customer line items.

System Response

Error message

How to fix this error?

You should either divide your document into several sub-documents, or
use a document type which is not designed for net posting.

Error message extract from SAP system. Copyright SAP SE.