View tag description on page

Your $tag_id variable is not defined before use. Try a test with a fixed value in there:

<p class="klientWlasciciel">WLASCICIEL: <?php echo 'Tag Description: ' . tag_description( '1' ); ?> </p>

… or test the default behavior which should be the current queried tag:

<p class="klientWlasciciel">WLASCICIEL: <?php echo 'Tag Description: ' . tag_description(); ?> </p>

https://codex.wordpress.org/Function_Reference/tag_description