When I didn't get any output, I ran it in Ubuntu 18 on a Linode VPS. But I tried to generate different formats: do, qcow, vmware.
Strange that there was no output or even errors.
BTW: I believe NixOS image generation can be run only on NixOS machine, because it needs to run a VM and that requires some kernel modules to be loaded. At least I never was able to made it work on OS X.
The nixos-infect code though will turn your Ubuntu installation into NixOS, so you might try to boot to your Ubuntu machine, convert it to NixOS and I'm hoping then nixos-generate might start working.
Note though that "do" didn't work either for me, and I didn't look to investigate why, I figured out that maybe it was because I was running it on machine on VirtualBox and not DO.
But I just tried, and it did work for me. The generated image was provided as the last line of the output.
It looked like this:
What OS you run it on? I run this on NixOS 20.03 on VirtualBox, I needed to have "Enable Nested VT-x/AMD-V" since the process spins a qemu to finalize the image.