Per Allan Sandfeld Jensen, there is some kind of DBus configuration for KScreen that is required to make it work. Normally packagers handle this for distributions. For those using kdesrc-build to compile their own KDE, this step needs to be replicated somehow. kscreen doesn't do this (presumably since it's a distro task?). kdesrc-build could do it. Maybe kdesrc-build /should/ do it? Or maybe instead it should be a separate module dedicated to just these two files (workspace-dbus-support?), which kdesrc-build could then install as root? See also the kde-devel thread at https://kde.markmail.org/thread/ggpryietqbb7i5nr
Closing as I believe this has been better addressed by updates to the KDE Community wiki instructions on setting up the development environment for Plasma.
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🐛🧹 This bug has been in NEEDSINFO status with no change for at least 30 days. Closing as RESOLVED WORKSFORME.