you have to take SSL licence from your web hoster for yor site
After installation if that will not work then try to add below code in your function.php of theme for particular page with ssl
add_action('wp_head', 'apply_ssl_to_specific_page');
add_action('admin_head', 'apply_ssl_to_specific_page');
add_action('login_head', 'apply_ssl_to_specific_page');
function apply_ssl_to_specific_page(){
<?php
if ( is_page('your-checkout-page-slug') ) {
if($_SERVER['SERVER_PORT'] != '443')
header('Location: https://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
} else {
if($_SERVER['SERVER_PORT'] == '443')
header('Location: http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
}
}
replace your-checkout-page-slug with your page slug