Means of transport combination in FO &1 not defined in Customizing

What causes this issue?

You planned a trailer unit on a freight order with a means of transport
combination that has not been defined in Customizing.

System Response

The system does not consider this means of transport combination in an
optimizer run. It can only be used within manual planning.

How to fix this error?

You can do the following:
Plan the trailer unit on the freight order manually
Create the means of transport combination in Customizing and perform the
optimizer run again

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.