Display parent page URL for child page

$post->post_parent returns 0 when there’s no parent associated. You need to check if the value is greater than 0.

<?php global $post;
  if ( $post->post_parent > 0 ) { ?>
    <a href="<?php echo get_permalink( $post->post_parent ); ?>" >
    <?php echo get_the_title( $post->post_parent ); ?>
    </a> <Br><?php echo get_permalink( $post->post_parent ); ?>
<?php } ?>

Also, check your permalink settings URL base.