This document cannot be added Error

We are consolidating various sales orders to a draft invoice using the Document Generation Wizard. At the end of the consolidation process, 1 draft invoice is created, but the following error shows in Orange:

Certain field values in the target document may differ from their value in the base documents [Message 131-185]

Then when we open the draft invoice and try to add it, we get the following error:

This document cannot be added, because the base document row was updated. Copy the document again from its base document. [Message 439-197]

Thanks and any help appreciated.

Adele