IF taxonomy archive is hierarchical THEN

Look at the docs (please read the docs) for is_taxonomy_hierarchical(). You need to tell it which taxonomy you’re checking:

if ( is_taxonomy_hierarchical( 'my_taxonomy_name' ) ) {


If you’re template isn’t specific to a taxonomy, and you need to know which taxonomy you’re viewing, use get_queried_object() to figure it out (you were already told how to do this, by the way):

if ( is_tax() ) {
    $taxonomy = get_queried_object()->taxonomy;

    if ( is_taxonomy_hierarchical( $taxonomy ) ) {

    } else {