Vanilla Rails 6.0: "error Command "webpack" not found" : <h2>System:</h2>

<p>Ruby: 2.6.3p62 (rvm)<br>
Rails: 6.0<br>
OS: macOS 10.14.6</p>

<h2>Setup</h2>

<p>A fresh Rails 6.0 application:</p>

<pre><code>$ rails new testshop2
$ cd testshop2
$ rails g controller Page index
$ rails s
=&gt; Booting Puma
=&gt; Rails 6.0.0 application starting in development 
=&gt; Run `rails server --help` for more startup options
Puma starting in single mode...
* Version 3.12.1 (ruby 2.6.3-p62), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3000
Use Ctrl-C to stop
</code></pre>

<p>When I browse to `<a href="http://localhost:3000/page/index" rel="noreferrer">http://localhost:3000/page/index</a>' the system throughs this error:</p>

<pre><code>Started GET "/page/index" for ::1 at 2019-09-23 17:06:12 +0200
  (0.4ms)  SELECT sqlite_version(*)
Processing by PageController#index as HTML
  Rendering page/index.html.erb within layouts/application
  Rendered page/index.html.erb within layouts/application (Duration: 1.8ms | Allocations: 206)
[Webpacker] Compiling…
[Webpacker] Compilation failed:
error Command "webpack" not found.

Completed 500 Internal Server Error in 2021ms (ActiveRecord: 0.0ms | Allocations: 640080)



ActionView::Template::Error (Webpacker can't find application in /Users/stefan/Github/sandbox/testshop2/public/packs/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
  unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
Your manifest contains:
{
}
):
    6:     &lt;%= csp_meta_tag %&gt;
    7: 
    8:     &lt;%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %&gt;
    9:     &lt;%= javascript_pack_tag 'application', 'data-turbolinks-track': 'reload' %&gt;
    10:   &lt;/head&gt;
    11: 
    12:   &lt;body&gt;

app/views/layouts/application.html.erb:9
</code></pre>

<p>How can I fix this? It says <code>error Command "webpack" not found.</code> but shouldn't Rails install everything needed automatically?</p>
