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.