Really Simple SSL is not an SSL certificate itself but a tool to configure your site for SSL. The plugin requires a valid SSL certificate before it can work correctly. How to install an SSL certificate differs from hosting provider to hosting provider, we recommend to contact your hosting provider for instructions on how to install the certificate.
you can check here: https://ssllabs.com/ssltest if you have a valid SSL certificate. If it returns a grade A, and the plugin still can’t detect the certificate, you should be able to force this by loading your backend over https.