"Your migration failed checking source credentials. Code: accessDenied Message: The caller does not have permission to perform the action. Inner error"
Slow sync between the back-end Group and SharePoint permissions causes the administrator’s access attempt to fail. Most commonly, this is caused by delays in replication on the Office 365 side for recently granted permissions. This could also indicate that the Admin being used for the migration is not an owner of the Team(s) in question.
Check to confirm that the Admin that is being used for the migration is listed as an owner of the Team that is experiencing this error. If not, add the Admin as an owner and then attempt the migration again. If the Admin is already showing as an owner, and was only recently made an owner, wait a couple hours to allow the permissions to fully propagate and then try the migration again.
Alternatively, skip the affected Team/Channel using a filter. For information on filtering a specific Team or Channel, see MigrationWiz – Collaboration Migration – Using a CSV to select which teams to migrate