使用以下代码，运行错误，未能创建 SSL/TLS 安全通道。如何处理

Dim name As String="张三"
Dim id As String="abc"
Dim msg As String = "username=" & name &"&idcard=" & id 
msgbox(msg)
Dim req = System.Net.WebRequest.Create("https://certificate/query_json.xhtml")
req.Method = "POST"
req.Timeout = 5000
req.ContentType = "application/x-www-form-urlencoded; charset=UTF-8"
req.UserAgent = "Mozilla/5.0 (Windows NT 6.3; rv:66.0) Gecko/20100101 Firefox/66.0"
Dim aryBuf As Byte() = Encoding.GetEncoding("utf-8").GetBytes(msg)
req.ContentLength = aryBuf.Length
Dim writer = req.GetRequestStream()
writer.Write(aryBuf, 0, aryBuf.Length)
writer.Close()
writer.Dispose()
Dim pos = req.GetResponse
Dim stm As System.IO.Stream = pos.GetResponseStream()
Dim reader As New System.IO.StreamReader(stm)
Dim str As String = reader.ReadToEnd
pos.Close
stm.Close
reader.close

msgbox(str)