How do I backup Nextcloudpi / Nextcloud to NAS?

Hi, everyone. I had my Nextcloudpi become unavailable from the web several weeks ago after a power outage (I posted about that here: Nextcloud not working since a power outage ). I have had very little time to try to back it up since the last reply to that question. When I tried backing up to my NAS, nothing happened, but I got this output:

I am very new to Linux, so not sure what I am missing or doing wrong. I have searched this forum for instructions on backing up to a NAS, but the posts I found referenced other pages that were no longer in existence.

I was unable to get help fixing the problem I had after the power outage, so I’m trying to back up whatever I can in such a way that my files could be restored to my Nextcloudpi if I end up having to completely reinstall it. It seems to me that this would be a common enough task that step-by-step instructions, easy to follow for a new user, would be included in a wiki or some such, but I have not been able to find anything of the sort.

I would really appreciate any help, and please keep in mind that I am very new to this. Thank you!

Steps taken to attempt backup:

-Directories created on 12TB NAS: ncp-bu/Backup

-In Nextcloudpi GUI, under “nc-backup,”:
Destination directory: //
Include data (for test purposes): Unchecked
Compress: Unchecked
Number of backups to keep: 1

After Applying, I got the output in the paste above. Backup directory empty.

Thank you for any suggestions.