Run the below command to remove the mailbox from the first migration batch in the list:
Remove-MigrationUser -Identity <MailboxIdentity> -BatchId 'FirstBatch'
Repeat this step for each migration batch that the mailbox is in.
Once you have removed the mailbox from all migration jobs, you can try migrating it again.
Some helpful articles - How to Troubleshoot Failed Migrations?
Please Note: Since the web sites are not hosted by Microsoft, the links may change without notice. Microsoft does not guarantee the accuracy of this information.