Cannot Retrieve User: The user does not exist or is not unique.: <username>
This error usually occurs when performing a migration from File System to either SharePoint or OneDrive. It happens when the files that were originally shared with a user that cannot be found at the Destination, either because the admin account was not assigned a username or because the username was incorrect, so the permissions failed to be applied.
An easy workaround is to create a user account at the Destination with the correct username, or with the the name Administrator, and then resubmit the migration.