For those who may find this later, after .NET version 4.6, I was running into this problem as well.
Make sure that you check your web.config file for the following lines:
<compilation debug="true" targetFramework="4.5">
...
<httpRuntime targetFramework="4.5" />
If you are running 4.6.x or a higher version of .NET on the server, make sure you adjust these targetFramework values to match the version of the framework on your server. If your versions read less than 4.6.x, then I would recommend you upgrade .NET and use the newer version unless your code is dependent on an older version (which, in that case, you should consider updating it).
I changed the targetFrameworks to 4.7.2 and the problem disappeared:
<compilation debug="true" targetFramework="4.7.2">
...
<httpRuntime targetFramework="4.7.2" />
The newer frameworks sort this issue out by using the best protocol available and blocking insecure or obsolete ones. If the remote service you are trying to connect to or call is giving this error, it could be that they don't support the old protocols anymore.