If you have this error when doing the Quickstart, then above all other recommendations on the internet, check the version of the Hugo as it may be old. The new one uses the hugo.toml as a default configuration file, that’s the one mentioned in Quickstart:

echo "theme = 'ananke'" >> hugo.toml

But, for the old one, you need to save it to the config.toml.

See more here.