If you are in the process of creating a Blazor app in Visual Studio 2019 that you installed right after its launch (around April 2019 timeframe), chances are you’ll find the checkbox titled “Use previews of the .NET Core SDK” in the following location:
Tools -> Options -> Projects and Solutions -> .NET Core
However, if you’ve installed Visual Studio 2019 recently (as of this writing), the option has moved to the following location:
Tools -> Options -> Environment -> Preview Features
Once the above is done, close Visual Studio IDE and open it again.
Now, Create a New Project and select ASP.NET Core Web Application template and hit ‘Next’, give a project name and hit ‘Next’ again and you’ll be presented with dialog with the option to select ASP.NET Core 3.0. Make sure you also select .NET Core on the LHS. This is a requirement for the successful creation of a Blazor app. (see image below).
Hope you found this post useful!