Here the -v /wherever/st-sync:/var/syncthing \ is where you basically connect a folder outside the Docker container to a folder inside the Docker container.
/wherever/st-sync would be some folder on your NAS and /var/syncthing the folder inside your Docker container that Syncthing uses.
So what happens is when you add a new folder in your GUI, for example /var/syncthing/test_folder it will be linked to /wherever/st-sync/test_folder. But if you add it as /var/test_folder it won’t be reachable outside the container. You really have to keep track of those volumes you set when starting a Docker container.
And what you seem to try to do now is change the latter (/var/syncthing) to something else (/mnt/user), first of all, that point doesn’t seem to be writable, secondly…it is probably not a linked volume.
So in short; double-check the volumes you have set (or did not set, then you’ll have to fix that part).