I’m getting this issue: the type or namespace name ‘Mvc’ does not exist in the namespace ‘Microsoft.AspnetCore’ (are you missing an assembly reference?).

While upgrading Blazor applications, you many encounter these issues. You have to do the following changes to properly run the application:

• Update all Microsoft.AspNetCore.Blazor.* package references to 3.0.0-preview4-19216-03.
• Remove any package reference to Microsoft.AspNetCore.Components.Server.
• Remove any DotNetCliToolReference to Microsoft.AspNetCore.Blazor.Cli and replace with a package reference to Microsoft.AspNetCore.Blazor.DevServer.
• In client Blazor projects, remove the <RunCommand>dotnet</RunCommand> and <RunArguments>blazor serve</RunArguments> properties.
• In client Blazor projects, add the <RazorLangVersion>3.0</RazorLangVersion> property.
• Rename all _ViewImports.cshtml files to _Imports.razor.
• Rename all remaining .cshtml files to .razor.
• Rename components.webassembly.js to blazor.webassembly.js
• Remove any use of the Microsoft.AspNetCore.Components.Services namespace and replace with Microsoft.AspNetCore.Components as required.
• Update server projects to use endpoint routing.

// Replace this:
app.UseMvc(routes =>
{
    routes.MapRoute(name: "default", template: "{controller}/{action}/{id?}");
});

// With this:
app.UseRouting();

app.UseEndpoints(routes =>
{
    routes.MapDefaultControllerRoute();
});

• Run dotnet clean on the solution to clear out old Razor declarations.

Reference link : Go through the upgrade section https://devblogs.microsoft.com/aspnet/blazor-now-in-official-preview/. You can also check this thread for more information.