SslProtocols does not contain a definition for Tls12

I get this Error when I import Watson SDK

Assets/Watson/Scripts/Connection/WSConnector.cs(416,72): error CS0117: `System.Security.Authentication.SslProtocols' does not contain a definition for `Tls12`

Does anyone know how to correct this?

As indicated in the documentation

Tls12 is a value of the SslProtocols enum available for .Net4.6. You will have to select this compatibility level in the Project Settings > Other Settings > Scripting Runtime Version of your Unity project.