The email of a shared mailbox should not need to use a license, either by Microsoft or by Quest Software On Demand Migration (ODM).
What may be going on is that mailbox is a shared mailbox on the source tenant but that it also exists on the target tenant and it is not a shared mailbox there. Then since it is a shared mailbox ODM doesn't see a license and when it goes to the target it doesn't try to give it a license for the target tenant. And without that license the migration fails.
It could also be that the _source_ tenant mailbox is configured as just a regular (not shared) mailbox afterall.