Impossible to log-in using v4/api

Troubleshooting
8

Sure! Here is what i use :
on RESTClient :
Method POST URL : https://Mattermost-URL/api/v4/users/login
Body : {
“login_id”:“user”,
“password”:“pw”
}

And the answer :

{
    "id": "api.user.login.use_auth_service.app_error",
    "message": "Please sign in using gitlab",
    "detailed_error": "",
    "request_id": "zdnkdp3k6pb53g3rcwpaik53nr",
    "status_code": 400
}