Please, note the following points:–
The Exchange account should be active.
To migrate multiple mailboxes, the account you are using should have full access rights over the mailboxes. If you don’t have, you can grant the full access rights to the mailbox from - https://www.nucleustechnologies.com/grant-full-access-to-all-mailboxes-in-office-365.html.
While restoring (importing), the PST files should be healthy. If your PST files are corrupt, you need to repair them first.