How display node author roles at node.html.twig
You can get roles in twig using:

{%  for role in node.Owner.roles %}
    {{ role.entity.label }}
{% endfor %}


Update :
If you want to use roles in classes i recommended you to use clean_class filter to get the clean css class from roles instead of the code you post in comment use this:

   {% set custom_classes = [] %}
    {% for role in node.Owner.roles %}
      {% set class_role = role.entity.label | clean_class %}
      {% set custom_classes = custom_classes | merge([class_role]) %}
    {% endfor %}
    &lt;div {{ attributes.addClass(custom_classes) }}&gt;
      {{ content }}
    &lt;/div&gt;