What was the message exactly? It should be one among those listed here: nethserver-upgrade-tool/root/usr/share/nethesis/NethServer/Language/en/NethServer_Module_UpgradeTool.php at master · NethServer/nethserver-upgrade-tool · GitHub
BTW: upgrading to AD is mandatory if you have Samba shared folders with user authentication in ns6.
This seems from the backup-data procedure. You’d check it is configured correctly or disable it temporarily. More information can be found in /var/log/messages.