The client and server cannot communicate, because they do not possess a common algorithm - ASP.NET C# IIS TLS 1.0 / 1.1 / 1.2 - Win32Exception

There are several other posts about this now and they all point to enabling TLS 1.2. Anything less is unsafe.

You can do this in .NET 3.5 with a patch.
You can do this in .NET 4.0 and 4.5 with a single line of code

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // .NET 4.5
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; // .NET 4.0

In .NET 4.6, it automatically uses TLS 1.2.

See here for more details: .NET support for TLS

In my case, even though Target Framework of Project was 4.7.1, I was still getting same Error, Solution was to change httpRuntime in web.config under system.web to 4.7.1!

In a previous answer, it was suggested to use this line of code for .Net 4.5:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // .NET 4.5

I would encourage you to OR that value in to whatever the existing values are like this:

ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12; // .NET 4.5

If you look at the list of values, you notice that they are a power of two. This way, in the future when things shift to TLS 2.0 for example, your code will still work.