I am trying to replicate this example with no success. I want to add a list using a mustache template, like this:

<ul>
    <amp-list width=auto
              height=100
              layout=fixed-height
              src="/assets/popular.json">
          <template type="amp-mustache"
                    id="amp-template-id">
              <li>
                  <a href={{url}}>{{title}}</a>
              </li>
          </template>
    </amp-list>
</ul>

My /assets/popular.json file is:

{
 "items": [
   {
     "title": "amp-carousel",
     "url": "https://ampbyexample.com/components/amp-carousel"
   },
   {
     "title": "amp-img",
     "url": "https://ampbyexample.com/components/amp-img"
   },
   {
     "title": "amp-ad",
     "url": "https://ampbyexample.com/components/amp-ad"
   },
   {
     "title": "amp-accordion",
     "url": "https://ampbyexample.com/components/amp-accordion"
   }
 ]
}

But I cannot get it to work, the values in the json are not being replaced in the template, I get this error:

Missing URL for attribute 'href' in tag 'a'

I do not know why the value {{url}} is not being replaced properly with the content of the json.

I've added the necessary scripts in the head.

Recently I've been migrating to Hugo from Jekyll and have faced the same issue. Below are solutions for both.

Jekyll

It is solved now, the problem was I am using jekyll, and so the tags {{tag}} were being interpreted as a liquid tag. I solved it writing the code like this:

<ul>
<amp-list width=auto
    height=100
    layout=fixed-height
    src="/assets/popular.json">
  <template type="amp-mustache"
      id="amp-template-id">
    <li>
      <a href="{% raw %}{{url}}{% endraw %}">{% raw %}{{title}}{% endraw %}</a>
    </li>
  </template>
</amp-list>
</ul>

Update: I have written a more detailed explanation

Hugo

<ul>
<amp-list width=auto
    height=100
    layout=fixed-height
    src="/assets/popular.json">
  <template type="amp-mustache"
      id="amp-template-id">
    <li>
      <a class="card related" id={{"{{id}}"}} {{ printf "href=%q" "{{url}}" | safeHTMLAttr }}>
         {{"{{title}}"}}
      </a>
    </li>
  </template>
</amp-list>
</ul>