Getting error while browsing the discovery service url although service is already installed and status is started
I think you'd need to change the security configuration to allow access to the discovery service.
Change Security OAuthEnabled to false and Rules Enabled to false in cd_ambient_conf.xml in the discovery service's config folder, and then restart the service.
That error can be considered a "good error"; if you get that error in your browser, it implies the Discovery Service is up-and-running and OAuth is enabled (so you will get such error unless you provide a valid bearer token in your request, which a browser will never do).
If you want to browse the contents of your Discovery Service, you can disable security (as described by Sabarish) or you could obtain a bearer token from the token service and send a request (e.g. using Postman or Fiddler) including the bearer token (which will expire shortly after you have requested it). Security is a necessary evil. :-)