If you have a namespaced route, like admin_post_path, you can’t use the resource-oriented style of form_for:

<% form_for @post do |f| %>
  ...
<% end %>

You need to do a little bit more work:

<% form_for([:admin, @post]) do |f| %>
  ...
<% end %>

(Source: api.rubyonrails.org)