Have you assigned the required permission in Exchange Online admin center?
For more details please refer to this similar thread:
The general steps are:
- Log into to ECP e.g. https://outlook.office365.com/ecp
- Select the Permissions menu, then Admin Roles
- Create a new role by clicking on the + sign
- Give it a name e.g. GSuiteMigration
- Click on the + under roles and add the ApplicationImpersonation & ViewOnly-Configuration
- Click the + sign under members and add the admin that requires the impersonation role
- Save and rerun the migration steps
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