Oauth access token Request is missing required authentication credential while inserting event into google calendar android - google-oauth

Question

When I am inserting the events into the Google calendar of my google account but it was throwing the 401 unauthorized error.
I am using the following code to add events to the calendar.
Event _calendarEvent = new Event()
.setSummary(_calendarData.get(0).get_calenderTitle())
.setDescription("AppName");
CommonUtility.convertDateFormat(_calendarData.get(0)
.get_startTime(), "yyyy-MM-dd HH:mm",
"yyyy-MM-dd'T'HH:m:ssZZZZZ");
DateTime startDateTime = new DateTime(CommonUtility.convertDateFormat(_calendarData.get(0).get_startTime(), "yyyy-MM-dd HH:mm", "yyyy-MM-dd'T'HH:mm:ssZZZZZ"));
EventDateTime start = new EventDateTime()
.setDateTime(startDateTime)
.setTimeZone(tz.getID());
start.setDateTime(startDateTime);
_calendarEvent.setStart(start);
DateTime endDateTime = new DateTime(CommonUtility.convertDateFormat(_calendarData.get(0).get_endTime(), "yyyy-MM-dd HH:mm", "yyyy-MM-dd'T'HH:mm:ssZZZZZ"));
EventDateTime end = new EventDateTime()
.setDateTime(endDateTime)
.setTimeZone(tz.getID());
end.setDateTime(endDateTime);
_calendarEvent.setEnd(end);
//401 unauthorized exception thrown by the insert method below.
Event _eventLogged = mService.events().insert(_credential.getSelectedAccountName(), _calendarEvent).execute();
The I am facing is shown below.
{
"code": 401,
"message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
"errors": [
{
"message": "Login Required.",
"domain": "global",
"reason": "required",
"location": "Authorization",
"locationType": "header"
}
],
"status": "UNAUTHENTICATED"
}
Authorization code
HttpTransport transport = AndroidHttp.newCompatibleTransport();
JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
mService = new Calendar.Builder(
transport, jsonFactory, credential)
.setApplicationName("AppName")
.build();
GoogleCredential code
mCredential = GoogleAccountCredential.usingOAuth2(
getApplicationContext(), Arrays.asList(SCOPES))
.setBackOff(new ExponentialBackOff());