Error operation not supported

Verified that this works fine on another PC connected the nas running 14.04.

Not my askubuntu question, so not the only one experiencing this.

http://askubuntu.com/questions/760631/transmission-and-deluge-fail-in-ubuntu-16-04-lts-saving-to-external-usb-hard-dri

Status changed to 'Confirmed' because the bug affects multiple users.

I am experiencing the same issue. The download to SMB share via transmission starts and then errors out with the same error you are facing. I wasn't facing this in 14.04. I am facing this only in 16.04

Same issue here on Ubuntu 16.04.

It appears to be a problem with permissions. I fixed the problems by changing the "rpc-username" and "rpc-password" fields in "~/.config/transmission/settings.json" to my own username and password.

never mind, that did not work

Workaround that works for me is mounting NAS through /etc/fstab (or just mount -t cifs <servername> <username> ...).
https://ubuntuforums.org/showthread.php?t=2309866
https://wiki.ubuntu.com/MountWindowsSharesPermanently

PS: I had a problem with write access to the mount point with my special credentials. It was solved by using guest user:
//servername/sharename /media/windowsshare cifs guest,uid=1000,iocharset=utf8 0 0
It is strange because such permissions are set for this share: read/write for everyone.

I encountered this issue when mounting via the 'mount -t cifs' command, but when I added an fstab entry and mounted it that way, the issue no longer occurred.

Hi all. I had the same issue with permissions and Transmission (Debian 9). Automounted cifs share in Gnome Nautilus works perfectly everywhere except Transmission.

Solved this issue by manually mounting with options:

sudo mount -t cifs //server/share /mountpath -o user=cifs_user,password=cifs_pw,uid=1000,gid=1000,iocharset=utf8

uid and gid belongs to desktop user you logging in and can be obtained by command:

id

or

cat /etc/passwd