Error

This is happening because your server has PHP V < 5.3. If this a private server you should update the server, if not in file [ERP plugin folder]/easy_related_posts.php lines 75-77 you now see

add_action('widgets_init', function () {
    register_widget("ERP_Widget");
});

change these three lines with the following code

function regERPWidget() {
    register_widget("ERP_Widget");
}
add_action('widgets_init', 'regERPWidget');