Hi,
The account you run ADMT under will need to have administrative rights in both the source and target domain.
Or there will be access denied error.
For more information, you can refer to the following links:
https://social.technet.microsoft.com/wiki/contents/articles/13904.how-to-migrate-users-across-forest-cross-forest-using-admt-3-2-with-sid-and-passwords.aspx
https://blog.thesysadmins.co.uk/admt-series-1-preparing-active-directory.html
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Best Regards,