This is really an issue with task-xfce-desktop, which recommends light-locker.
XFCE 4.20 dropped light-locker in favour of xfce4-screensaver (which was added to Trixie). Users installing Trixie XFCE are running into this issue, and the solution seems to be to uninstall light-locker and install xfce4-screensaver. https://forums.debian.net/viewtopic.php?p=827120&hilit=light+locker#p827120 There is a bug report against task-xfce-desktop: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1034607
