Unfortunately, this is an issue with Teaspoon and newer (>4.0) versions of Rails. For some reason, the asset paths for Teaspoon are no longer included in the list of assets that are automatically precompiled. Therefore, you must add them to your config/initializers/assets.rb file like so:
Error: Sprockets::Rails::Helper::AssetFilteredError: Asset filtered out and will not be served: add `Rails.application.config.assets.precompile += %w( teaspoon.css )` to `config/initializers/assets.rb` and restart your server
Note: I added a surrounding check for production. Since Teaspoon is a testing gem, you shouldn't be using it in production, nor should you need its assets. If you have assets in this file that you need for production, you can separately add them with another addition to Rails.application.config.assets.precompile.
unless Rails.env.production? Rails.application.config.assets.precompile += %w( teaspoon.css teaspoon-teaspoon.js teaspoon-mocha.js # other assets )