List subcategory on taxonomy term page

You can make use of get_term_children() instead of get_categories()

You can try something like this

$term = get_queried_object();
$term_id = $term->term_id;
$taxonomy_name = $term->taxonomy;

$termchildren = get_term_children( $term_id, $taxonomy_name );

echo '<ul>';
foreach ( $termchildren as $child ) {
    $term = get_term_by( 'id', $child, $taxonomy_name );
    echo '<li><a href="' . get_term_link( $term, $taxonomy_name ) . '">' . $term->name . '</a></li>';
}
echo '</ul>';