$ cat configuration.nix
{
  "kshaa" = { config, pkgs, ... }: {
    imports = [
      ./modules/users.nix
    ];

    services = {
      openssh = {
        enable = true;
        gatewayPorts = "yes";
      };
    };
  };
}

$ cat virtualbox.nix
{
  "kshaa" = { config, pkgs, ... }: {
    deployment.targetEnv = "virtualbox";
    deployment.virtualbox.memorySize = 1024;
    deployment.virtualbox.vcpu = 1;
    deployment.virtualbox.headless = true;
  };
}

$ nixops deploy -d kshaa_virtualbox --show-trace
building all machine configurations...
error: while evaluating the attribute 'buildCommand' of the derivation 'nixops-machines' at /nix/store/ydni1jrlb137ndr7s79l62x2rgyrpkpd-nixpkgs-19.09pre173445.796a8764ab8/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
while evaluating anonymous function at /nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/share/nix/nixops/eval-machine-info.nix:369:46, called from undefined position:
while evaluating the attribute 'config.system.build.toplevel' at /home/kshaa/.nix-defexpr/channels/nixpkgs/nixos/modules/system/activation/top-level.nix:274:5:
while evaluating 'foldr' at /home/kshaa/.nix-defexpr/channels/nixpkgs/lib/lists.nix:39:20, called from /home/kshaa/.nix-defexpr/channels/nixpkgs/nixos/modules/system/activation/top-level.nix:138:12:
while evaluating 'fold'' at /home/kshaa/.nix-defexpr/channels/nixpkgs/lib/lists.nix:42:15, called from /home/kshaa/.nix-defexpr/channels/nixpkgs/lib/lists.nix:46:8:

Failed assertions:
- Mountpoint '/': 'autoResize = true' is not supported for 'fsType = "auto"': fsType has to be explicitly set and only the ext filesystems and f2fs support it.
Traceback (most recent call last):
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/bin/..nixops-wrapped-wrapped", line 990, in <module>
    args.op()
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/bin/..nixops-wrapped-wrapped", line 411, in op_deploy
    max_concurrent_activate=args.max_concurrent_activate)
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/lib/python2.7/site-packages/nixops/deployment.py", line 1056, in deploy
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/lib/python2.7/site-packages/nixops/deployment.py", line 1045, in run_with_notify
    f()
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/lib/python2.7/site-packages/nixops/deployment.py", line 1056, in <lambda>
    self.run_with_notify('deploy', lambda: self._deploy(**kwargs))
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/lib/python2.7/site-packages/nixops/deployment.py", line 996, in _deploy
    self.configs_path = self.build_configs(dry_run=dry_run, repair=repair, include=include, exclude=exclude)
  File "/nix/store/fi0w3msyy982han8yqlb0pn6ffzwc04p-nixops-1.6.1/lib/python2.7/site-packages/nixops/deployment.py", line 664, in build_configs
    raise Exception("unable to build all machine configurations")
Exception: unable to build all machine configurations