I suspect chromium-browser has not been installed properly. In the past this has been the case due to a certificate problem on the server where the chromium-browser package is downloaded from. But that has been resolved and should not be the issue now. There have also been problems to download from certain countries (e.g. China) which block some internet traffic… Could this have happened in your case, too?
Apart from that: The chromium-browser version that should have been installed by the plugin is 56.0.2924.84. If you followed the steps described in this post you will likely have version 48.0.2564.82.
To investigate further please post the results of
chromium-browser -version
systemctl status -l volumio-kiosk.service
and
ls -al /tmp/.X11-unix
Changes to rc.local are not required to run the touch display plugin. I would recommend to revert them.
Did you install any other display related software like light-dm?