It looks like the home function in your routes.py file is defined as part of a Flask blueprint named main. When using a blueprint, you need to prefix the endpoint name with the name of the blueprint when generating URLs using url_for.
In your home.html file, you are generating URLs for the home endpoint without prefixing the name of the blueprint. To fix this error, you can update the url_for calls to use the main.home endpoint instead of just home:
{% extends "layout.html" %}
{% block content %}
{% for post in posts.items %}
<article class="media content-section">
<img class="rounded-circle article-img" src="{{ url_for('static', filename='profile_pics/' + post.author.image_file) }}">
<div class="media-body">
<div class="article-metadata">
<a class="mr-2" href="{{ url_for('user_posts', username=post.author.username) }}">{{ post.author.username }}</a>
<small class="text-muted">{{ post.date_posted.strftime('%Y-%m-%d') }}</small>
</div>
<h2><a class="article-title" href="{{ url_for('post', post_id=post.id) }}">{{ post.title }}</a></h2>
<p class="article-content">{{ post.content }}</p>
</div>
</article>
{% endfor %}
{% for page_num in posts.iter_pages(left_edge=1, right_edge=1, left_current=1, right_current=2) %}
{% if page_num %}
{% if posts.page == page_num %}
<a class="btn btn-info mb-4" href="{{ url_for('main.home', page=page_num) }}">{{ page_num }}</a>
{% else %}
<a class="btn btn-outline-info mb-4" href="{{ url_for('main.home', page=page_num) }}">{{ page_num }}</a>
{% endif %}
{% else %}
...
{% endif %}
{% endfor %}
{% endblock content %}
In the code above, the url_for('main.home') calls have been updated to include the name of the blueprint (main) before the endpoint name (home). This should fix the BuildError that you're seeing.
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