Hi, what kind of “monitor combinations” have you been trying? 🙂
In any case, depending on the video card and drivers, it’s possible that the GPU is turned off on a headless system, so in that case, screen capture would also be impossible. On previous versions of Gnome that still support(ed) X11, NoMachine would be able to create a virtual display in these specific cases, but this won’t work with the version of Gnome that is present in Ubuntu 26.04. Running pure Wayland virtual desktops will be added in NoMachine 10, so watch out for that. For now you could consider using a different desktop environment or attach a dummy HDMI dongle.