Remove Menu Page Giving Error

Put the add_action line inside the if (!current_user_can('activate_plugins') ) {:

<?php
/************ Remove admin menu items from anyone who isn't an admin ************/

if (!current_user_can('activate_plugins') ) { 
    function my_remove_menu_pages() {
        remove_menu_page('link-manager.php');
        remove_menu_page('tools.php');
        remove_menu_page('edit-comments.php');
    }

    add_action( 'admin_menu', 'my_remove_menu_pages' );
};
?>