@Gokulnathkumar your problem here is not finding the user name. The ID you see is likely not your user at all, but lack of user. Here is the scenario that would cause similar problem: you create artifact feed, but don't add your agent to the list of contributors. And that's it, nuget will fail to restore packages, saying that this user can't get access. The ID will not be the ID of your agent.
Check if your agent is properly registered, i.e. you see user "Agent Pool Service (#)", where # is ID of your agent. You may want to modify permissions of PAT used and restart your agent.