Different transportation management system for freight order &1

What causes this issue?

You assigned freight documents from different transportation management
systems to an invoice:
Document &V3& is from system &V4&; however, document &V1& is from system
&V2&.

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?

Check the assignment and delete assigned documents or deselect
individual invoice items if necessary.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.