At this time you should add term in little bit another way (in compare with [this][1] answer)
First of all in your file begin you should write 

> use Drupal\taxonomy\Entity\Term;

Because Term class listed in Drupal\taxonomy\Entity.
And you don't need to pass taxonomy_term parametr to this method, because only one parametr is needed (array with values) (below listed code for this method in taxonomy module)

    public function create(array $values = array()) {
    // Save new terms with no parents by default.
    if (empty($values['parent'])) {
      $values['parent'] = array(0);
    }
    $entity = parent::create($values);
    return $entity;
  }

  [1]: http://drupal.stackexchange.com/questions/108868/programmatically-create-term-in-drupal-8#answer-108874