Hello Hrishi,
Since Cloudera Navigator cannot authenticate this specific user, the issue may lie with its inability to find the user in Active Directory.
You should compare the location of the user in Active Directory with Navigator's External Authenication "LDAP User Search Base" property.
The user's AD location may not be within the scope of the "LDAP User Search Base" property. If that is the case, you may need to widen the property's scope.