Encountering ErrorIntenarlServerError when creating an event using Graph API - microsoft-graph-api

Question

I Was able to create an event on a not delegated calendar but failed on the delegated calendar when using this endpoint:
https://graph.microsoft.com/v1.0/me/calendars/{calendarId}/events
I also tried to put a wrong access token and calendarId on the request just to make sure if I can encounter the same error message but It didn't trigger the issue.
This is the object we're passing.
CalendarRequest
{
start = new MicrosoftDateTimeZoneRequest() {
dateTime = dateTimeOffset.ToString(dateTimeFormat),
timeZone = "Pacific Standard Time"
},
end = new MicrosoftDateTimeZoneRequest() {
dateTime = dateTimeOffsetEnd.ToString(dateTimeFormat),
timeZone = "Pacific Standard Time"
},
subject = $"{activity.CustomerName} {activity.ActivityName} {activity.ActivityType}".Trim(),
body = new MicrosoftItemBodyRequest() { content = activity.Details + (activity.CustomerId > 0 ? ("|" + clientURL) : ""), contentType = "HTML" },
location = new MicrosoftLocationRequest() {
displayName = "Home"
},
attendees = new List<MicrosofAttendeeRequest>() {
new MicrosofAttendeeRequest() {
emailAddress = new MicrosoftEmailAddressRequest(){address = "karlo#tapnztest.onmicrosoft.com", name = "X TAP" },
type = "resource"
}
},
allowNewTimeProposals = false,
isOrganizer = true
}
And this is how I call the endpoint
var calendarResponse = GetCalendarEvent(accessToken);
var calendarID = calendarResponse.value.Where(a => a.owner.address == "test#tapteam.co.nz").FirstOrDefault()?.id;
var addCalendarEventRequest = new RestClient($"https://graph.microsoft.com/v1.0/me/calendars/{calendarID}/events");
var request = new RestRequest();
request.AddHeader("Authorization", "Bearer " + accessToken);
request.AddJsonBody(calendarRequest);
var response = addCalendarEventRequest.ExecuteAsync(request, Method.POST);
Payload
{
"subject": "CC - 1st Meeting Confirmed CC - 1st Meeting Confirmed",
"body": {
"content": "test",
"contentType": "HTML"
},
"start": {
"dateTime": "2021-02-25T15:30:00.000+13:00",
"timeZone": "Pacific Standard Time"
},
"end": {
"dateTime": "2021-02-25T16:00:00.000+13:00",
"timeZone": "Pacific Standard Time"
},
"location": {
"address": null,
"coordinates": null,
"displayName": "Home",
"locationEmailAddress": null,
"locationUri": null,
"locationType": null
},
"attendees": [
{
"emailAddress": {
"address": "karlo#tapnztest.onmicrosoft.com",
"name": "Karlo TAP"
},
"proposedNewTime": null,
"status": null,
"type": "resource"
}
],
"allowNewTimeProposals": false,
"transactionId": null,
"isOrganizer": true
}