EDI/SOA Invoice not Consistent

What causes this issue?

In an IDoc or SOA message, the delivery items must either all be invoice
items, or they must all be credit memo items.
In the first case, if all delivery items represent invoice items, all
returns items in this IDoc or SOA message must be credit memo items. In
the second case, if all delivery items represent credit memo items, all
returns items in this IDoc or SOA message must be invoice items.

System Response

The invoice cannot be posted correctly.

How to fix this error?

Use separate invoices for deliveries and returns.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.