I get the error “The requested security protocol is not supported” even though tls is turned on

Question

I am working on a soap webservice application written with .net 3.5 framework. An external service we use has changed and I updated it, but even though I opened everything related to TLS on a windows 2012 R2 machine, I get The requested security protocol is not supported” error.

• I created client server key for TLS 1.2 and 1.1 via registry editor and entered their values

• SchUseStrongCrypto and SystemDefaultTlsVersions values were assigned as 1. both under wow6432Node and flat microsoft (although I selected v2 from the application pool for the .net 3.5 framework, I set it in v4 just in case)

• I turned on TLS 1.0 1.1 1.1 1.2 over internet protocol

• I made my additions on the code

  public const SslProtocols _Tls12 = (SslProtocols)0x00000C00; 
  public const SecurityProtocolType Tls12 = (SecurityProtocolType)_Tls12;  
  //The following part is in the function I use
  ServicePointManager.SecurityProtocol = Tls12;
  ServicePointManager.ServerCertificateValidationCallback = delegate (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
            {
                return true;
            };
  

  works fine on other windows server 2016 machines I installed . What am I missing?