There is a fatal error in your site when installing Cornerstone. To resolve this, please provide us you FTP. What you have given is just your WP credentials. The FTP details is needed so that we can check your theme and plugin files.
You can place the ftp details in the secure note when you reply.
Thanks for updating in! I have logged in and investigated the issue. I found out that you are using WordPress 4.2.2 and the latest version is 5.2.2. This is why there is an error in your admin page. The wp_roles() function is a default WordPress function release in version 4.3 that we use for our theme.
To resolve your issue, please update your WordPress core to the latest version.