Site5.com does not support free LetsEncrypt certificates and only offers paid SSL certificates with their hosting plans. This means you do not use the Really Simple SSL wizard to install an SSL certificate on your website. Really Simple SSL will still assist you in making sure all your content is served over SSL en properly configure other relevant SSL & security settings.
You will need to order an ssl certificate and install it manually. The installation instructions depend on the type of hosting panel your site uses. Contact site5 support for help with ssl installation.