Not sure what is causing this since every prerequisites have been installed!

It likely has nothing to do with the modules installed.

Typically in this context, 500 is an HTTP 500 error -- which means, the webserver you're connecting to threw an error. You should look at the webserver's error logs.

In your particular case, it also specifically says '500 connect failed', which may mean that the SOAP client wasn't able to connect to the webserver. You should also make sure that the URL you're trying actually works. (and because you're using a service, make sure that you can get to the proxy that's mentioned in the WSDL, too)