UPDATE: Colin\'s suggestion of removing the line //= require_tree . has fixed the issue.
I have wasted over 2 days trying to follow every suggestion
For beginners like me:
Change line 6 from:
'<%= javascript_include_tag 'application', 'data-turbolinks-track' => true %>'
to
<%= javascript_include_tag 'defaults', 'data-turbolinks-track' => true %>
Source from tutorial to fix here