External Links are not opening . It is showing 

{"error":"Sorry, User is not available"}

URL : https://safalaya.com/blog/view/930866/how-to-find-owner-of-a-public-forum-website-in-india

Please help urgently as my users are facing inconvenience .

External Link? From other sites to your site or from your site to other sites? In the first case it might be that page (or content on the page) can only be viewed with sufficient permission (i.e. the user needs to be logged in to your site and might have to be also a member of a group). If links on your site to other sites are not working I would think it might be rather a problem on these other sites (non-existent pages, invalid urls?).

By default elgg is not packed with push notifications. If you are seeing them in your site, some plugin or theme you are using is causing this. 

I have removed sw.js file from root folder . Please check again on all devices .

No. We don't need to check anything. YOU should check what's sending these push notification on your Elgg site. As Sarath said, Elgg sends no push notification without any additional plugin installed that provides this functionality. Disable this plugin and the push notifications should stop. I don't know what the sw.js file should be. But removing any files without knowing their purpose can easily break the site. So, you might think before installing any stuff you don't need and then asking each day a new question about something not working on your site.