Block definitions should be at the top level, not the included level

templates/forms/inputs/user-name.html.twig

@@ -1,4 +1,3 @@
-{% block input_user_name %}
 {% if 'name' not in form.fields.hidden %}
     {% if col_width %}<div class="{{col_width}}">{% endif %}
         <div class="row">
@@ -6,5 +5,4 @@
             {% include "forms/inputs/user-last_name.html.twig" with { 'col_width': 'col-sm-6' } %}
         </div>
     {% if col_width %}</div>{% endif %}
-{% endif %}
-{% endblock %}
+{% endif %}