In my case, I was running under Visual Studio 2022. Time and time again I was getting this error. Going through the code I saw that it retrieved the certificate just fine. Security was set to TLS1.2, both answers above. For whatever reason, running Visual Studio as Administrator made it work! Maybe someone can explain to me how the code retrieved the certificate from the store just fine. I could see it and all the properties. Why in the name of this world would it not process the request unless I run VS in admin mode???

这是原始答案所没有的。我又加了些代码让它防弹。

ServicePointManager.Expect100Continue = true;
        ServicePointManager.DefaultConnectionLimit = 9999;
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12 | SecurityProtocolType.Ssl3;

我有这个问题是因为我的网。配置:

<httpRuntime targetFramework="4.5.2" />

而不是:

<httpRuntime targetFramework="4.6.1" />

这个错误是一般的，有很多原因导致SSL/TLS协商失败。最常见的是无效或过期的服务器证书，您可以通过提供自己的服务器证书验证钩子来解决这个问题，但这并不一定是唯一的原因。服务器可能需要相互身份验证，它可能配置了一组客户端不支持的密码，它可能有太大的时间漂移，以至于握手无法成功，还有很多其他原因。

最好的解决方案是使用channel故障排除工具集。channnel是负责SSL和TLS的SSPI提供者，您的客户端将使用它进行握手。看看TLS/SSL工具和设置。

另请参阅如何启用通道事件日志记录。

如果服务器对HTTP请求返回HTTP 401未授权响应，则会出现“请求已终止:无法创建SSL/TLS安全通道”异常。

您可以通过打开跟踪级系统来确定是否发生这种情况。Net日志记录用于客户端应用程序，如本回答中所述。

一旦日志配置就绪，运行应用程序并重现错误，然后在日志输出中查找如下一行:

System.Net Information: 0 : [9840] Connection#62912200 - Received status line: Version=1.1, StatusCode=401, StatusDescription=Unauthorized.

在我的情况下，我未能设置服务器所期望的特定cookie，导致服务器以401错误响应请求，从而导致“无法创建SSL/TLS安全通道”异常。

这一个是为我工作在MVC webclient

public string DownloadSite(string RefinedLink)
{
    try
    {
        Uri address = new Uri(RefinedLink);

        ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;

        System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

        using (WebClient webClient = new WebClient())
        {
            var stream = webClient.OpenRead(address);
            using (StreamReader sr = new StreamReader(stream))
            {
                var page = sr.ReadToEnd();

                return page;
            }
        }

    }
    catch (Exception e)
    {
        log.Error("DownloadSite - error Lin = " + RefinedLink, e);
        return null;
    }
}