The settings page says redirect could not be set in the .htaccess

Your site can run on SSL without any issues. The plugin also adds some javascript to redirect any non https pages, so your site should load over https without any problems.
Furthermore, you can enable the HSTS setting to improve security.

Common causes:
1. The .htaccess is not writable, or not available.
2. Testing of the .htaccess rewrite rules failed. This can be caused by several things:

  • The plugin couldn’t access the test page: some configurations do not allow access to the plugin from a URL.
  • Your server does not pass a server variable that indicates the SSL protocol. This is needed if you want to redirect all requests in .htaccess.
Lightweight plugin, Heavyweight Security features. Get Pro and leverage your SSL certificate for WordPress security standards.