Assign delivery for TPI-relevant orders to TD shipments?

What causes this issue?

Do you really want to manually assign deliveries for TPI-relevant
orders, to TD shipments?
Note: The external TPI system plans these deliveries and sends an IDoc
to assign them to the TD shipment.

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

System Response

How to fix this error?

In the SAP Oil & Gas System, you can not change subsequent
documents of orders that have been sent to the transportation planning
system or that have been created by the transportation planning system.
In exceptional cases, for example when the transportation planning
system is not available, you can configure this block function in
Customizing for the Industry Solution Oil & Gas (Downstream). You
can set the message that the system displays to be either an error
message, a warning message, an information message, or you can switch
it off.
To make this setting, choose TD (Transportation and Distribution) ->
TPI
(Transportation Planning Interface) -> Configure Message Handling.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.