User details can not update via rest api

Hello,

I am using JIRA rest api JIRA 7.0.2. In that trying to update user information by using below method(as per documentation-https://docs.atlassian.com/jira/REST/latest/#api/2/user-updateUser):
http(s)://jiraEndpointUrl/rest/api/latest/user?username=User8
passing below json:

{ "name": "IRISUser8", "emailAddress": "irisuser8@gmail.com", "displayName": "IRIS User8" }

when I am calling with above details then it will return below error(json response) to me with status code=400.
Json response(error):-
{"errorMessages":["This user can not be renamed because that operation is unsupported by the underlying user directory or JIRA is acting as a Crowd Server."],"errors":{}}.

So can anyone please let me know why it will give me an error for that and whats wrong in my API Code

Thanks