Recently for a console application we got the below error while using CrmServiceClient.
The possible reason is we haven’t specified that security protocol TLS12 to be used in our console application and it is still targeting .NET Framework 4.5.2.
Add the following line of code to get it working if that is the case.
If we are using Visual Studio 2017, simply change the Target Version to .NET Framework 4.6 or higher. It internally defaults to Tls12, so we do not need to specify it explicitly.
Hope it helps..