It was the crazy couple of month ago when all payment gateway did enforce the security standard to TLS 1.2
There were systems which were built in .Net Framework 4.0 and enforcing TLS 1.2 was a bit tricky there
Here is the one-liner that fixes TLS 1.2 Issue.
ServicePointManager.SecurityProtocol = (SecurityProtocolType) 3072;
We are all set 🙂