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

https://blogs.msdn.microsoft.com/crm/2017/09/28/updates-coming-to-dynamics-365-customer-engagement-connection-security/

How we can fix it à

We need to add the following line of code to our existing code

Helpful post

https://debajmecrm.com/2018/01/08/fixed-error-while-connecting-to-dynamics-365-version-9-0-metadata-contains-a-reference-that-cannot-be-resolved/

https://waelhamze.wordpress.com/2018/01/11/dynamics-365-ce-tls-connectivity-issue-from-net-and-powershell/

http://abhinavranjan.xyz/2017/12/21/issue-authenticating-crm-organization-service-web-app-july-2017-update/

Hope it helps..