You may have encountered the following error message when trying to mount a CIFS share from the web interface of your Proxmox server. If you run the following command below via SSH, you will get a little more detail.

❯ pvesm add cifs pooter-linux-isos --server 192.168.2.9 --share linux-isos --username proxmox --password
Enter Password: *********
mount error(95): Operation not supported
create storage failed: error with cfs lock 'file-storage_cfg': mount error: Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)

The issue is solved by running the command with –smbversion 2.0 or 2.1 or 3.0 as per this forum topic. For my Synology it was –smbversion 2.0 that worked.

https://forum.proxmox.com/threads/cifs-issue-error-with-cfs-lock-file-storage_cfg-working-now-but-shows-question-mark.45962/