I've reproduced your problem. I installed
Fedora-Workstation-Live-x86_64-25-1.3.iso on a system, then did;
sudo dnf groupinstall sugar-desktop.
Starting Terminal and typing
sugar did not work. Prompt returned with no error, but exit status 1. Looking at log file
.sugar/default/logs/shell.log there is a missing package, which could be fixed with;
sudo dnf install webkitgtk3
Caused by the Fedora package of Sugar not declaring a dependency. After start,
sugar did work once.
But on logout gdm would not show a Sugar desktop entry, and the GNOME desktop would no longer work. It would not start, as you saw.
After applying all updates with
dnf update, the login screen gdm itself would not work.
You might take this further with Fedora. It looks much too fragile to be used.