Comment 27 for bug 211966

Still affected in 16.04.6 upgraded from Trusty : the log in this machine shows

Mar 28 17:41:43 nux gnome-session[25837]: Nautilus-Share-Message: Called "net usershare info" but it failed: « net usershare » returned error 255 : mkdir failed on directory /var/run/samba/msg.lock: Permission denied
Mar 28 17:41:43 nux gnome-session[25837]: net usershare: cannot open usershare directory /var/lib/samba/usershares. Error None file or directory of this kind
Mar 28 17:41:43 nux gnome-session[25837]: Please ask your system administrator to enable user sharing.

even after I created /var/lib/samba/usershares and touch /var/run/samba/msg.lock (*):

~ ls -l /var/lib/samba/
total 8
drwxr-xr-x 3 root root 4096 march 4 2017 private
drwxr-xr-x 2 root root 4096 march 28 17:42 usershares

# members sambashare
me

$ ls -l /var/run/samba/msg.lock
-rw-r--r-- 1 root root 0 mars 28 18:55 /var/run/samba/msg.lock

(*)Maybe I should have mkdir msg.lock ?
Yes I should:
$ net usershare info --long
directory /var/run/samba/msg.lock isn't a directory

# rm /var/run/samba/msg.lock
# mkdir /var/run/samba/msg.lock

Wait and see

Here only nautilus-share is installed (I was prompted to install it by Files one day when I wanted to share a folder - Samba not installed : not prompted=no need)