无法在Azure中使用系统分配的托管身份读取Azure Key Vault秘密值

Question

遵循的步骤：

使用启用的身份创建新的VM（已分配系统）
使用存储帐户中的角色分配在IAM中添加了VM
能够使用C＃生成令牌
对于现有VM，获取托管身份服务。
[使用-az keyvault set-policy --name“” --object-id“” --secret-permissions获得权限分配访问天蓝色KV的权限列表
现在从VM执行代码时出现错误

我能够从GetToken()方法获取令牌，但出现此错误：

Unhandled exception. System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   at System.Net.HttpWebRequest.GetResponse()
   at KeyFromAzKeyVault.Program.FatchSecretKeyFromKeyVault(String token)

CODE：

namespace KeyFromAzKeyVault
{
    class Program
    {
        static void Main(string[] args)
        {
            var token = GetToken();
            Console.WriteLine(token);
            Console.WriteLine("*************FatchSecretKeyFromKeyVault from KV*********************");
            string key = FatchSecretKeyFromKeyVault(token);
            System.Console.WriteLine(key);
        }

        private static string FatchSecretKeyFromKeyVault(string token)
        {
            WebRequest kvrequest = WebRequest.Create("https://testkvaz203.vault.azure.net/secrets/testsecret/65c8c3b1e56e4a2fae0b6c8033c6b9ce?api-version=7.0");
            kvrequest.Headers.Add("Authorization", "Bearer " + token);
            WebResponse response = kvrequest.GetResponse();
            return ParseWebResponse(response, "value");
        }

        private static string GetToken()
        {
            WebRequest request = WebRequest.Create("http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01&resource=https://vault.azure.net/");
            request.Headers.Add("Metadata", "true");
            request.Method = "GET";
            WebResponse response = request.GetResponse();
            return ParseWebResponse(response,"access_token");
        }

        private static string ParseWebResponse(WebResponse response, string tokenName)
        {
            string token = string.Empty;
            using (Stream stream = response.GetResponseStream())
            {
                StreamReader streamResponse = new StreamReader(stream, Encoding.UTF8);
                string stringResponse = streamResponse.ReadToEnd();
                JObject JoResponse = JObject.Parse(stringResponse);
                JValue Jobject = (JValue)JoResponse[tokenName];
                token = Jobject.Value.ToString();
                return token;
            }
        }
    }
}