ACF simple text field value not showing

please check your postype and add arg

<?php 
$query = new WP_Query( array( 'post_type' => 'ourproducts' ) );
if ( $query->have_posts() ) :
    while ( $query->have_posts() ) : 
        $query->the_post(); 
?>
        <h3><?php echo get_field('nasi_proizvodi'); ?></h3>
<?php 
    endwhile; 
    wp_reset_postdata(); 
endif; 
?>