Error 500 – internal server error

Hello, I’m using Zerif-lite theme. When I’m trying to enter on my page, getting error 500. The problem comes from functions.php. There is a row with function:

/* Load Jetpack compatibility file. */

require get_template_directory() . ‘/inc/jetpack.php’;

function zerif_wp_page_menu() {
echo ‘<ul class=”nav navbar-nav navbar-right responsive-nav main-nav-list”>’;
wp_list_pages(array(‘title_li’ => ”, ‘depth’ => 1));
echo ‘‘;
}

add_filter(‘the_title’, ‘zerif_default_title’);
function zerif_default_title($title) {
if ($title == ”)
$title = __(“Default title”,’zerif-lite’);
return $title;
}

Guess there is a problem with jetpack installation files but if I delete that function from functions.php and enter on my page there will be just a white “Blank Page”.

How can I fix this?

Thank you