Restricting frontend acess based on user role otherwise redirect to login form

WP can’t redirect after you have already output HTML – so move your if/else to the top, before you call for the header, and only output the header & footer if your condition is met.

Also, instead of using current_user_can('role'), use current_user_can('capability') – i.e.

<?php if(
    current_user_can('activate_plugins') ||
    current_user_can('edit_a_defined_custom_post_type')
) {
    get_header(); ?>
    <div>Restricted content here</div><?php
    get_footer();
} else {
    wp_redirect(wp_login_url());
} ?>

(According to the Codex, “While checking against particular roles in place of a capability is supported in part, this practice is discouraged as it may produce unreliable results.”)