Post documents of type 'Internal' or 'External' in a single posting block

What causes this issue?

You have attempted to post a mixture of internal and external documents
in the same posting block. This is not permitted in the system.

System Response

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

How to fix this error?

You must cancel the current posting block and retry posting only
internal or only external documents within a single defined posting
block. A posting block containing both internal and external documents
will not run correctly.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.