I have a build configuration that was already producing .nupkg files. It does not use any of the NuGet TeamCity build steps to do this, as it uses some MSBuild tasks to do so instead. The build configuration already includes the generated .nupkg files as build artifacts. I've also enabled the native NuGet Server option in the TeamCity server admin page.
The problem is that the NuGet channel URL published by TeamCity is still empty. From the documentation I've read, it seems the only real requirement is that .nupkg files are reported as artifacts and that the nuget server is enabled, and the rest happens "magically". But since that's not working, does anyone know what step I'm missing?