You have this line:
= simple_form_for task, class: 'clearfix' do |f|
that looks like it’s opening a block, but there is nothing indented below it to be the blocks contents.
You just need to correctly indent your code:
.modal-header
%h1 New Task
= simple_form_for task, class: 'clearfix' do |f|
.modal-body
= f.input :title
= f.input :note
= f.input :completed
.modal-footer
= f.submit 'Save', class: 'btn btn-primary'