Doubt with bitly and c #

I'm trying to shorten urls with bitly using the c# console, the problem is that I get the error:

Error: System.Net.WebException: The remote server returned an error: (403) Forbidden.
   at System.Net.HttpWebRequest.GetResponse()
   at BitlyTest.Program.Shorten(String groupId, String token, String url) in Program.cs:line 54

This is the code:

class Program{
        static void Main(string[] args){
            Console.WriteLine("Enter group Id:");
            string groupId = Console.ReadLine();
            Console.WriteLine("Enter token:");
            string token = Console.ReadLine();
            Console.WriteLine("Enter URL:");
            string url = Console.ReadLine();
            Shorten(groupId, token, url);
        }

        public static void Shorten(string groupId, string token, string url){
            string post = "{\"group_guid\": \"" + groupId + "\", \"long_url\": \"" + url + "\"}";
            string shortUrl = url;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://api-ssl.bitly.com/v4/shorten");
            try{
                System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) { return true; };
                request.ServicePoint.Expect100Continue = false;
                request.Method = "POST";
                request.ContentLength = post.Length;
                request.ContentType = "application/json";
                request.Headers.Add("Cache-Control", "no-cache");
                request.Host = "api-ssl.bitly.com";
                request.Headers.Add("Authorization", "Bearer " + token);
                using (Stream requestStream = request.GetRequestStream()){
                    byte[] postBuffer = Encoding.ASCII.GetBytes(post);
                    requestStream.Write(postBuffer, 0, postBuffer.Length);
                }using (HttpWebResponse response = (HttpWebResponse)request.GetResponse()) //línea con el error
                {
                    using (Stream responseStream = response.GetResponseStream()){
                        using (StreamReader responseReader = new StreamReader(responseStream)){
                            string json = responseReader.ReadToEnd();
                            shortUrl = Regex.Match(json, @"""link"": ?""(?<link>[^,;]+)""").Groups["link"].Value;
                        }
                    }
                }
            }catch (Exception ex){
                Console.WriteLine("Error: " + ex);
            }
            Console.WriteLine("\nShortened URL: " + shortUrl);
            Console.ReadKey();
        }
    }

Could someone help me? I left the line in which the error appears commented out just in case. I have tried to solve the error by reading but I have not found the solution. The idea is then to be able to implement this idea in C# with MVC in a WebService. I hope you can help me, thanks.