Try using the partials explicitly:
views/tasks/index.html.erb
6:<div class="tasks" id="incomplete_tasks">
7: <%= render partial: 'task', collection: @incomplete_tasks %>
11:<div class="tasks" id="complete_tasks">
12: <%= render partial: 'task', collection: @complete_tasks %>
and
views/tasks/create.js.erb
3:$('#incomplete_tasks').append('<%= j render(partial: 'task', object: @task) %>');