Trouble exporting outlook calendar using angular

I am working on a angular web app which is a in house solution for developers to handle vacations (employees request vacations/sick days, admins can approve or deny request and so forth).

What I'm having trouble with is configuring a feature that handles sending calendar data to Outlook using vsto

In angular I set this up outlook-calendar.service.ts:

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { Observable, BehaviorSubject } from 'rxjs';
import { ReturnHandler } from '../modules/vacation/Models/ReturnHandler';
import { OutlookCalendar } from '../modules/vacation/Models/outlookCalendar';

@Injectable({
  providedIn: 'root'
})

export class OutlookCalendarService {
  httpOptions = {
    headers: new HttpHeaders({ 'Content-Type': 'application/json' })
  };
  
  private OutlookUrl = '/api/OutlookCalendar';
  private outlookBH$: BehaviorSubject<number> = new BehaviorSubject<number>(0);
  public outlook$: Observable<number> = this.outlookBH$.asObservable();

  constructor(private http: HttpClient) { }
  
  AddOutlookAppointment(outlookCalendar: OutlookCalendar) : Observable<ReturnHandler> {
    const url = `${this.OutlookUrl}/CreateAppointment`;
    return this.http.post<ReturnHandler>(url, outlookCalendar, this.httpOptions);
  }

  DeleteOutlookAppointment(outlookCalendar: OutlookCalendar) : Observable<ReturnHandler> {
    const url = `${this.OutlookUrl}/DeleteAppointment`;
    return this.http.post<ReturnHandler>(url, outlookCalendar, this.httpOptions);
  }

  OutlookYearCalendarGenerator(year: number) : Observable<ReturnHandler>{
    const url = `http://localhost:50304/vac/api/OutlookCalendar/CreateNewOutlookCalendar?year=${year}`;
    //const url = `${this.OutlookUrl}/CreateNewOutlookCalendar?year=${year}`;
    console.log(year);
    return this.http.get<ReturnHandler>(url, this.httpOptions);
  }

  // WriteOutlookNationalHolidays(year: string) : Observable<ReturnHandler>{
  //   const url = `${this.url}/WriteOutlookNationalHolidays?year=${year}`;
  //   return this.http.get<ReturnHandler>(url,this.httpOptions);
  // }
}

In visual studio, ThisAddIn.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Outlook = Microsoft.Office.Interop.Outlook;
using Office = Microsoft.Office.Core;
using System.Windows.Forms;
using System.IO;

namespace OutlookCalendar
{
    public partial class ThisAddIn
    {
        private OutlookCalendarCs outlookCalendar;

        protected override object RequestComAddInAutomationService()
        {
            if(outlookCalendar == null)
                    outlookCalendar = new OutlookCalendarCs();

            return outlookCalendar;
        }

        private void ThisAddIn_Startup(object sender, EventArgs e)
        {
            Outlook.NameSpace session = Globals.ThisAddIn.Application.Session;
            Outlook.Accounts accounts = session.Accounts;
            List<Outlook.MAPIFolder> calendars = new List<Outlook.MAPIFolder>();

            foreach (Outlook.Account account in accounts)
            {
                Outlook.Recipient recipient = session.CreateRecipient(account.DisplayName);
                Outlook.MAPIFolder calendar = session.GetSharedDefaultFolder(recipient, Outlook.OlDefaultFolders.olFolderCalendar);
                if (calendar != null)
                {
                    calendars.Add(calendar);
                }
            }

            File.WriteAllLines("dd.txt", (IEnumerable<string>)calendars);

            Console.WriteLine(calendars);

            using (TextWriter tw = new StreamWriter("C:\\AxessCroTools\\IIS\\dd.txt"))
            {
                foreach (Outlook.MAPIFolder calendar in calendars)
                {
                    tw.WriteLine(calendar);
                }
            }
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
            // Note: Outlook no longer raises this event. If you have code that 
            //    must run when Outlook shuts down, see https://go.microsoft.com/fwlink/?LinkId=506785
        }
        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
        
        #endregion
    }
}

Now when pressing my generate outlook calendar button I encounter this error:

Http failure response for http://localhost:50304/vac/api/OutlookCalendar/CreateNewOutlookCalendar?year=undefined: 0 Unknown Error