If you run x.py build in a directory with no config.toml, it suggests you run x.py setup first.
I did that.
Then I ran x.py build again. And it made the same suggestion.
The suggestion wasn't wrong, since there was no config.toml in my current directory, even after running x.py setup; the problem was that running x.py setup is generating the config.toml in the source root, rather than in the current directory.