Hi Kim,
You will need to untick the box Backup data to compressed archive (.rkn) file.
For more detail info on this, please view the document from the link below
"There are no attachments available message when restoring a backup in ReckonAccounts 2013."
http://kb.reckon.com.au//issue_view.asp?ID=4739
Even though the document relates to ReckonAccounts 2013, this can be applied to ReckonAccounts 2015
Regards
Dean