I think, you're mistaken in how `if` works...?

	$post_meta = get_post_meta(
		$GLOBALS['wp_query']->post->ID,
		'TourURL',
		true
	);
	if (! empty($post_meta)) echo '<p>Show Link</p>';
	else echo '<p>No Link</p>';

This should do what you want - if I understood you correctly.

**// UPDATED**