dub: Could not resolve configuration for package demo

[mw via Digitalmars-d-learn]

Very simple thing from https://dub.pm/getting-started/first-steps/

After I add dependencies, it cannot build:

```
$ cat dub.json
{
        "authors": [
                "mw"
        ],
        "copyright": "Copyright © 2024, mw",
        "description": "msgpack-rpc-d demo.",
        "license": "MIT",
        "name": "demo",
        "dependencies": {
                "msgpack-rpc": "~>0.1.3"
        },
        "subConfigurations": {
                "msgpack-rpc": "default"
        }
}

$ dub build
Could not resolve configuration for package demo
```

So what I'm missing?

Thanks.