Failed to generate portfolio gallery for Jekyll website

Question

I am facing troubles to auto-generate Jekyll gallery for my portfolio.html file. I have stored portfolio landing pages in _clients folder on site root. The images for these landing pages are stored in {{site.baseurl}}/images/clients folder.

Here is my code for autogenerating portfolio gallery:

<section>
<div class="container">
<ul class="image-grid">
            {% for client in site.clients %}
                <li>
                    <a href="{{ site.baseurl }}{{ client.url }}">
            <img src="{{ site.baseurl }}/images/clients/{{ client.image_path }}" class="img-responsive" alt="{{ post.alt }}">
           <div class="details">
                            <div class="name">{{ client.name }}</div>
                            <div class="position">{{ client.subtitle }}</div>
                        </div>
                    </a>
                </li>
            {% endfor %}
            <li class="filler"></li>
            <li class="filler"></li>
        </ul>
    </div>
</section>

Now. here is my code for generating landing page when someone clicks on gallery image:

<section> 
<div class="container">
    {% if portfolio.heading %}
    <h2>
      <a href="{{ site.baseurl }}/portfolio/">{{ portfolio.heading }}</a> /
      {{ page.title | capitalize }}
    </h2>
    {% endif %} {% if page.subtitle %}
    <p class="subtext">{{ page.subtitle }}</p>
    {% endif %}
  </div>
</section>

<section>
 <div class="container">
    <p>
      <img
        src="{% include relative-src.html src=page.image_path %}"
        class="screenshot"
      />
    </p>
    <div>
      {{ content }}
    </div>
    <p>
      <a href="{{ page.external_url }}">View {{ page.title }} &rarr;</a>
    </p>
  </div>
</section>

Here's how font yml of client.md file looks like:

---
name:
subtitle:
external_url:
image_path:
---