What causes this issue?

You have changed the <DS:GLOS.partner>partner</> assignment in the
order.
From the partner assignment of the order, the system determines the
default values for determining delivery addresses of external
resources.
You may have to adapt the delivery addresses of the external resources
according to the partner change.

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 delivery addresses of the external resources and redefine
them if necessary.

Error message extract from SAP system. Copyright SAP SE.