Unfortunately, I don’t get that far. When the package is about to be installed, the error message appears. Activating the service via systemctl does not work because the package (lightdm) cannot be installed and the mentioned error message appears.
Gnome and gdm was just an example that it worked with it, but didn’t have it installed.
If you installed GNOME from the RebornOS installer, lightDM is already installed on your system, so there is no need to do it again. You just have to deactivate gdm and activate lightDM.
In the case of changing gdm to lightdm if we use GNOME:
You have to disable gdm, and enable lightdm. To do this, from the terminal: