Sprockets::FileNotFound in FileName#index

Sprockets::FileNotFound in #index

下記エラー

// about supported directives.
//
//= require rails-ujs <-- これが無いとのこと
//= require turbolinks
//= require_tree .

なので、削除します。

app/assets/javascripts/application.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives./= require rails-ujs
//
                     <- ここにあったものを削除しました。
//= require turbolinks
//= require_tree .

これで表示されます。