Hello @Sudhanshu Vudaru
Welcome to the community.
To be able to mention users you must be allocated the "Browse Users and Groups" permission, which is a permission allocated at the Global level by Jira Administrators. Contact your Jira Administrators and ask them to confirm that you are a member of a user group to whom that permission has been granted.
Reference: https://support.atlassian.com/jira-cloud-administration/docs/manage-global-permissions/
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