get_field() not working after recent upgrade

Hi,

We just upgraded our plugin to latest version and seems that the get_field() has stopped working. When I check the logs it says:

Call to undefined function get_field() in single-post.php on line 16

Here is the specific code I’m using:

foreach ( get_field( 'pictures' ) as $nextgen_gallery_id ) :
                $pictures_gallery_id = $nextgen_gallery_id['ngg_id'];
                $pictures_gallery = $nggdb->get_gallery($pictures_gallery_id, 'sortorder', 'ASC', true, 0, 0);
                if(count($pictures_gallery) > 0){ ?>
                    <div class="gal pictures">
                        <div class="gallery">
                        <?php foreach($pictures_gallery as $image) { ?>
                            <div class="slide">
                                <img src="<?php echo $image->imageURL; ?>">
                            </div>
                        <?php } ?>
                    </div>
                        <div class="arrows">
                            <a href="#" class="right_arrow"><img src="<?php echo get_template_directory_uri(); ?>-child/images/right_arrow.png"></a>
                            <a href="#" class="left_arrow"><img src="<?php echo get_template_directory_uri(); ?>-child/images/left_arrow.png"></a>
                        </div>
                        <div class="gallery_frame">
                            <div class="gallery_small">
                                <?php foreach($pictures_gallery as $image) { ?>
                                    <div class="slide">
                                        <img src="<?php echo $image->imageURL; ?>">
                                    </div>
                                <?php } ?>
                            </div>
                        </div>
                    </div>
                <?php }
            endforeach;

Any ideas or how I can fix this?