@Eric Henderson
One of the mail enabled security groups on the on-prem Exchange server has the same email address as one of the Microsoft Teams Team.
For Exchange on-premises group, it cannot be migrated, you need to remove from Exchange on-premises, then recreate it on Exchange online. Bue to this email address is used, so you cannot recreate it on Exchange online.
As AndyDavid said, when AAD Connect syncs, there will also exist issue due to duplicate email addresses. So, you need to remove one of them before sycing.
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