Fatal error on wordpress theme

I am using a wordpress theme.It works good but in the portfolio section its shows fatal error.
Here is the message

Fatal error: Call to undefined function get_field() in /home/wakefiel/public_html/wp-content/themes/wakefield/overview.php on line 50

The overview.php code is

<?php
/**
 * @package WordPress
 * @subpackage Default_Theme
 Template Name: Overview Template
 */

get_header(); ?>

<?php if ( have_posts() ) while ( have_posts() ) : the_post(); ?>
	<?php
				$page_id = null;
               if($post->post_parent):
                 $page_id = get_top_ancestor($post->ID);
               else:
                 $page_id = $post->ID;
                 endif;
				$parent_title = get_the_title(get_page($page_id));
		?>

				<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
					<?php if ( is_front_page() ) { ?>
                    	<?php
						  	$homeTitle = get_post_custom_values("home-title");
							if (isset($homeTitle[0])) {
								echo '<h1>' . $homeTitle[0] . '</h1>';
						?>
                        <?php } else { ?>
                            <h1 class="entry-title"><?php the_title(); ?></h1>
                        <?php } ?>
					<?php } else { ?>
						<h1 class="entry-title"><?php the_title(); ?></h1>
					<?php } ?>

					<div class="entry-content">
						<?php the_content(); ?>

						<?php wp_link_pages( array( 'before' => '<div class="page-link">' . __( 'Pages:', 'twentyten' ), 'after' => '</div>' ) ); ?>
						<?php edit_post_link( __( 'Edit', 'twentyten' ), '<span class="edit-link">', '</span>' ); ?>
					</div><!-- .entry-content -->

				</div><!-- #post-## -->

                 <?php if (get_field('overview')) { ?>
                 	<ul class='overview-list'>
                 <?php 	while(has_sub_field('overview')){ ?>
                  		 <li>
                         	<a href='<?php the_sub_field('link'); ?>'><h2><?php the_sub_field('title'); ?></h2>
                         	<img src='<?php the_sub_field('image'); ?>' />
                            </a>
                         </li>
                 <?php } ?>
                 	</ul>
                  <?php } ?>

				<?php //comments_template( '', true ); ?>

<?php endwhile; ?>

<?php get_footer(); ?>

Here is the website link https://www.wakefieldinc.ca/portfolio/

I am waiting for your good suggestion.
Thanks