When migrating the meeting messages, Migrator for Groupwise will get the calendar item UID through GroupWise Web Service, this parameter haven't been provided by Object API. The warning means that GroupWise Web Service is unavailable, but the SOAP URL was set.
All instances in the recurrence meeting have the same UID, Migrator For Groupwise will use this parameter to associate with the instances, if cannot get the UID, Migrator for Groupwise will use the creation time and subject and sender information to calculate the UID.
The item still get migrated with this warning. And the warning can be ignored. The SOAP need to be enabled on the Groupwise server if need to resolve this warning.