Blazor the type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?)

I have setup the basic application in Blazor in Microsoft Visual Studio Community 2019 Version 16.1.3 and I have tried both of the versions of .NET Core SDK 3.0.100-preview5-011568 and SDK 3.0.100-preview6-012264.

Also I have installed the Blazor extension but when I am building without any modification in any files, the build fails with the following error:

"The type or namespace "App" could not be found.

Any help will be highly appreciated. Thanks.