The TVHadmin-JS addon for Tvheadend cannot be installed in LibreElec the way it's described in the project wiki, see github project.
I've opened an issue for the project, where you can find some research on the root cause of the problem: in LibreElec the tvheadend web server along with the root path
/usr/share/tvheadend/src/webui/static
is built into the tvheadend binary and not accessible to the user.
That's where my capabilities end and I'm asking to support dave-p when he's willing to create a TVHadmin-JS addon for LibreElec. This would be really cool.