urls.py

from django.conf.urls import url
from django.contrib import admin
from django.contrib.auth import views as auth_views

from accounts import views as accounts_views
from boards import views

urlpatterns = [
    url(r'^$', views.home, name='home'),
    url(r'^signup/$', accounts_views.signup, name='signup'),
    url(r'^login/$', auth_views.LoginView.as_view(template_name='login.html'), name='login'),
    url(r'^logout/$', auth_views.LogoutView.as_view(), name='logout'),

    url(r'^reset/$',
        auth_views.PasswordResetView.as_view(
            template_name='password_reset.html',
            email_template_name='password_reset_email.html',
            subject_template_name='password_reset_subject.txt'
        ),
        name='password_reset'),
    url(r'^reset/done/$',
        auth_views.PasswordResetDoneView.as_view(template_name='password_reset_done.html'),
        name='password_reset_done'),
    url(r'^reset/(?P<uidb64>[0-9A-Za-z_\-]+)/(?P<token>[0-9A-Za-z]{1,13}-[0-9A-Za-z]{1,20})/$',
        auth_views.PasswordResetConfirmView.as_view(template_name='password_reset_confirm.html'),
        name='password_reset_confirm'),
    url(r'^reset/complete/$',
        auth_views.PasswordResetCompleteView.as_view(template_name='password_reset_complete.html'),
        name='password_reset_complete'),

    url(r'^settings/password/$', auth_views.PasswordChangeView.as_view(template_name='password_change.html'),
        name='password_change'),
    url(r'^settings/password/done/$', auth_views.PasswordChangeDoneView.as_view(template_name='password_change_done.html'),
        name='password_change_done'),

    url(r'^boards/(?P<pk>\d+)/$', views.board_topics, name='board_topics'),
    url(r'^boards/(?P<pk>\d+)/new/$', views.new_topic, name='new_topic'),
    url(r'^boards/(?P<pk>\d+)/topics/(?P<topic_pk>\d+)/$', views.topic_posts, name='topic_posts'),
    url(r'^admin/', admin.site.urls),
]

Hey Vitor!! I'm working through the series, loving it so far. Looks like this one doesn't have the updated path, so here it is!!

path('boards/int:pk/topics/int:topic_pk/', views.topic_posts, name='topic_posts'),

path('boards/int:pk/topics/int:topic_pk/', views.topic_posts, name='topic_posts'),

int:pk and topics;pk in brackets

This works for me !!