MarsSeed  filed a deletion request for fluxgui :
Project added warning in 2020 that most likely this won't work or won't properly work with modern systems. Lack of screen adjustment was reported as early as 2016 with Linux 4.4.
Developer recommends that people user RedShift instead.
My addition to that: major desktops like GNOME and KDE have already added night light dimming / yellowing of the screen. So this originally Windows application is not needed now on Linux. Most people don't even need to install anything nowadays for such functionality.
I think trying to fix this application is not worth the effort anymore.
Request #34985 has been Accepted by polyzen :
[Autogenerated] Accepted deletion for fluxgui.