Hello @Subash Kumar
Welcome to the community.
Have the users ever logged into Jira?
Are you working with a Company Managed project or a Team Managed project?
What field are you trying to update?
Have you been granted the global permission for Browse Users and Groups, by a Jira Administrator?
So, you can create an Automation for Jira rule that is based on the Issue transitioning to a particular status. Then you can set the assignee and reviewers automatically.
Here is some more information on Automation for Jira:
There are two types of automation:
Automation Basics: https://www.atlassian.com/software/jira/guides/expand-jira/automation
You can do this via global rules to apply to all projects also. You'll need to be a Jira Admin/Site Admin, go to Jira Settings > System > Automation Rules (left-hand menu).
For more on using Automation For Jira see these help pages.
Jira Automation Template Library to help get you started quickly: