I provided an answer in another similar question: https://stackoverflow.com/a/79761592/15891701

Here's a repeat of that answer:

Conclusion: Blazor server launched within WPF, behaves identically to a Blazor server project created directly in Visual Studio. This means that launching without a `launchSettings.json` file during debugging causes the “{PACKAGE ID/ASSEMBLY NAME}.style.css” file to generate fail. You can also notice this mirrors the effect of double-clicking an exe file in the Debug directory of an ASP.NET Core project.

So just create a launchSettings.json file in the Properties folder with content like this, and debugging will work correctly:

{
  "profiles": {
    "YourProjectName": {
      "commandName": "Project",
      "environmentVariables": {
        "ASPNETCORE_ENVIRONMENT": "Development"
      }
    }
  }
}