This error means that the inbox from your source account does not exist with the same name in Help Scout.
This occurs if you choose to map your mailboxes by matching on a field (such as Mailbox Name), as seen in this setup:
To fix the issue, you can either:
Make sure that your mailbox is named the same in Helpscout as in your source app
Reach out to customer success team to set up value mapping if you wish to map to a mailbox with a different name