I want to add PWA in existing Blazor Server Application.

Getting this error !!

You can manually add the files to your Blazor Server application to enable PWA functionality. Here are the steps to follow:

1. In your Blazor Server application project, create a new folder named "wwwroot".

2. Inside the "wwwroot" folder, create a new file named "service-worker.js".

3. Open the "service-worker.js" file and paste the following code:

   const CACHE_NAME = "my-pwa-cache";
   const urlsToCache = [
     "/",
     "/css/site.css",
     "/_framework/blazor.server.js",
     "/_framework/Microsoft.AspNetCore.Blazor.Browser.js",
     "/_framework/Microsoft.AspNetCore.Blazor.js",
     "/_framework/Microsoft.AspNetCore.Components.dll",
     "/_framework/Microsoft.AspNetCore.Components.Web.dll",
     "/_framework/Microsoft.Extensions.DependencyInjection.dll",
     "/_framework/Microsoft.Extensions.DependencyInjection.Abstractions.dll",
     "/_framework/Microsoft.JSInterop.dll",
     "/_framework/Microsoft.JSInterop.WebAssembly.dll",
     "/_framework/System.Net.Http.dll",
     "/_framework/System.Runtime.CompilerServices.Unsafe.dll",
     "/_framework/System.Text.Encodings.Web.dll",
     "/_framework/System.Text.Json.dll",
     "/_framework/webassembly.js"
   ];
   
   self.addEventListener("install", function (event) {
     // Perform install steps
     event.waitUntil(
       caches.open(CACHE_NAME).then(function (cache) {
         console.log("Opened cache");
         return cache.addAll(urlsToCache);
       })
     );
   });
   
   self.addEventListener("fetch", function (event) {
     event.respondWith(
       caches.match(event.request).then(function (response) {
         // Cache hit - return response
         if (response) {
           return response;
         }
         return fetch(event.request);
       })
     );
   });

    

Save the "service-worker.js" file. In your Blazor Server application project, open the "Pages/_Host.cshtml" file. Add the following line of code inside the element:

Sir, there is not any Service Worker in the item

And what about serviceworker.js file? 

Yes, you can add PWA (Progressive Web App) functionality to your existing Blazor Server application.

In Visual Studio 2022, right-click on your Blazor Server project and select "Add" > "New Item".
In the "Add New Item" dialog box, search for "Service Worker" and select "Service Worker" from the list of templates.
Name your Service Worker file and click "Add". This will add a new Service Worker file to your project.
Open the newly added Service Worker file and add the following code to it:

const cacheName = 'my-pwa-cache';

self.addEventListener('install', e => {
  e.waitUntil(
    caches.open(cacheName).then(cache => {
      return cache.addAll([
        '/',
        '/index.html',
        '/css/site.css',
        '/js/site.js'
      ]);
    })
  );
});

self.addEventListener('fetch', e => {
  e.respondWith(
    caches.match(e.request).then(response => {
      return response || fetch(e.request);
    })
  );
});

In your Blazor Server application's Startup.cs file, add the following code to the Configure method:

app.UseBlazorFrameworkFiles();
app.UseStaticFiles();
app.UseServiceWorker(new ServiceWorkerOptions
{
    ServiceWorkerPath = "/serviceworker.js"
});

This code adds the Service Worker file to your application's static files and sets the Service Worker path.