Use this instead:
https://extensions.gnome.org/extension/897/turn-off-display/
This is an extension that adds a button to gnome panel that when clicked
instantly turns off the monitor.
xset dpms force off is executed in
the backend.
Source: https://bitbucket.org/xtranophilist/gnome-shell-extension-screen-off
Installation Instructions:
Download:cd ~/.local/share/gnome-shell/extensions
git clone https://bitbucket.org/xtranophilist/gnome-shell-extension-screen-off.git
mv gnome-shell-extension-screen-off/ screen-off@xtranophilist
Restart Gnome:
Alt+F2 to open run command prompt
Enter r
Enable Screen Off Extension from Extensions from gnome-tweak-tool.