Microsoft Graph

I'm trying to create a meeting as a application, add attendees, determine time availability.

This is what I have so far :

Auth

private async Task<ClientCredentialProvider> GetToken()
{
    var confidentialClientApplication = ConfidentialClientApplicationBuilder
        .Create(_microsoftAppId)
        .WithTenantId(_microsoftTenantId)
        .WithClientSecret(_microsoftAppPassword)
        .Build();

    ClientCredentialProvider authProvider = new ClientCredentialProvider(confidentialClientApplication);
    return authProvider;
}
    

Meeting request

public async Task GetMeetingtime()
{
    var authProvider = await GetToken();
    GraphServiceClient graphClient = new GraphServiceClient(authProvider);

    var attendees = new List<AttendeeBase>()
    {
        new AttendeeBase
        {
            Type = AttendeeType.Required,
            EmailAddress = new EmailAddress
            {
                Name = "john doe",
                Address = "john.doe@onmicrosoft.com"
            }
        },

        new AttendeeBase
        {
            Type = AttendeeType.Required,
            EmailAddress = new EmailAddress
            {
                Name = "john toe",
                Address = "john.toe@onmicrosoft.com"
            }
        }
    };


    var locationConstraint = new LocationConstraint
    {
        IsRequired = false,
        SuggestLocation = false,
        Locations = new List<LocationConstraintItem>()
        {
            new LocationConstraintItem
            {
                ResolveAvailability = false,
                DisplayName = "Conf room Hood"
            }
        }
    };


    var timeConstraint = new TimeConstraint
    {
        ActivityDomain = ActivityDomain.Work,
        TimeSlots = new List<TimeSlot>()
        {
            new TimeSlot
            {
                Start = new DateTimeTimeZone
                {
                    DateTime = "2020-12-10T09:00:00",
                    TimeZone = "Pacific Standard Time"
                },
                End = new DateTimeTimeZone
                {
                    DateTime = "2020-12-10T17:00:00",
                    TimeZone = "Pacific Standard Time"
                }
            }
        }
    };

    var isOrganizerOptional = false;

    var meetingDuration = new Duration("PT1H");

    var returnSuggestionReasons = true;

    var minimumAttendeePercentage = (double)100;

    await graphClient
        .Me
        .FindMeetingTimes(attendees, locationConstraint, timeConstraint, meetingDuration, null, isOrganizerOptional, returnSuggestionReasons, minimumAttendeePercentage)
        .Request()
        .Header("Prefer", "outlook.timezone=\"Pacific Standard Time\"")
        .PostAsync();
}

This is the error I get:

Current authenticated context is not valid for this request. This occurs when a request is made to an endpoint that requires user sign-in. For example, /me requires a signed-in user. Acquire a token on behalf of a user to make requests to these endpoints. Use the OAuth 2.0 authorization code flow for mobile and native apps and the OAuth 2.0 implicit flow for single-page web apps.

How can I solve this?