screensaver
 

When I go to preferences/control center from my gnome start menu and and select screensaver I get

"There was an error launching the application.

Details: Failed to execute child process "xscreensaver-demo" (No such file or directory)"

However if I open it from the command line it works.

What is wrong?

Well, maybe Gnome is trying to show a demo of the screensaver you choosed, but it can't find the package "scree...-demo"

But it is possible your s.s. is working although it can't be "previewed" (?)