We would run into this issue while trying to connect to Dynamics 365 Version 9.0 from a client application.
It is because Customer Engagement Platform (version 9.0) only supports TLS 1.2.
More details here
How we can fix it à
We need to add the following line of code to our existing code

public static OrganizationServiceProxy GetOrganizationServiceProxy()
{
ClientCredentials clientCredentials = new ClientCredentials();
clientCredentials.UserName.UserName = "username";
clientCredentials.UserName.Password = "password";
// Set security protocol to TLS 1.2 for version 9.0 of Customer Engagement Platform
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
return new OrganizationServiceProxy(new Uri("https://orgname.crm.dynamics.com/XRMServices/2011/Organization.svc"),
null, clientCredentials, null);
}
Helpful post
Hope it helps..








