There are multiple reasons the Blazor application wouldn’t run in Visual Studio Code.
Scenario 1: The omnisharp Visual Studio Code is not installed properly in the machine to run the application. Refer to the getting started with Blazor using Visual Studio Code documentation.
Scenario 2: Running a Blazor app with F5 in Visual Studio is not supported in omnisharp, which is tracked in this GitHub issue.
For now, you can run the Blazor application using the following dotnet CLI command in the application root folder.