#include <gtk/gtk.h>
This occurs when the runtime cannot connect to the X11 server or Wayland abstraction layer. Ensure your DISPLAY variable is set correctly ( export DISPLAY=:0 ). If running on Windows via an X-server client like VcXsrv, verify that client access permissions are explicitly granted. gtk runtime environment version 2414
flatpak install flathub org.gtk.Gtk3runtime//2414 #include <gtk/gtk