If you use the Beautiful Jekyll theme on your Github blog, you can set the categories as follows.

Step 1

Edit the _config.yml file

...
...
...

# --- Navigation bar options --- #

# List of links in the navigation bar

navbar-links:
  Category:
    - TroubleShooting: "/category/troubleshooting"
    - Study: "/category/study"

...
...
...

Note: If a category is added, modify the _config.yml file again by adding a new category name.

Step 2

Create a /category directory and create an .md file with each category name.

2019-09-29-jekyll-theme-add-category_1.png

2019-09-29-jekyll-theme-add-category_2.png

The contents of the .md file are as follows.

---
layout: category
title: study
permalink: category/study
---

Note: If a category is added, add a .md file with the new category name.

Step 3

Create a category.html file in the / _layouts directory and write:

---
layout: default
---
<ul class="posts-list">  
  {% assign category = page.category | default: page.title %}
  <h4>Posts in {{ category }} ({{ site.categories[category].size }})</h4>
  {% for post in site.categories[category] %}   
    <li>
      <a class="post-title" href="{{ site.baseurl }}{{ post.url }}">{{ post.title }}</a>
      <small><time>
        {{ post.date | date:"%F" }} {{ post.date | date: "%a" }}.
      </time></small>
    </li>
  {% endfor %}
</ul>

Result

2019-09-29-jekyll-theme-add-category_3.png

2019-09-29-jekyll-theme-add-category_4.png