При наследовании родительский шаблон "base.html" не видит дочерних "navbar.html", "cardslider.html".
"base.html"
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.1/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-+0n0xVW2eSR5OomGNYDnhzAbDsOXxcvSN1TPprVMTNDbiYZCxYbOOl7+AMvyTG2x" crossorigin="anonymous">
<link rel="stylesheet" href="{{ url_for('static', filename='css/index.css') }}">
<link rel="stylesheet" href="{{ url_for('static', filename='css/cardslider.css') }}">
<title>Title</title>
</head>
<body>
{% block navbar %}{% endblock %}
{% block cardslider %}{% endblock %}
<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.1/dist/js/bootstrap.bundle.min.js" integrity="sha384-gtEjrD/SeCtmISkJkNUaaKMoLD0//ElJ19smozuHV6z3Iehds+3Ulb9Bn9Plx0x4" crossorigin="anonymous"></script>
</body>
</html>
"navbar.html"
{% extends "base.html" %}
{% block navbar %}
<nav class="navbar navbar-expand-lg navbar-dark bg-dark">
<div class="container-fluid">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNavDropdown" aria-controls="navbarNavDropdown" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNavDropdown">
<ul class="navbar-nav">
<li class="nav-item">
<a class="nav-link active" aria-current="page" href="#">Home</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Features</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Pricing</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdownMenuLink" role="button" data-bs-toggle="dropdown" aria-expanded="false">
Dropdown link
</a>
<ul class="dropdown-menu" aria-labelledby="navbarDropdownMenuLink">
<li><a class="dropdown-item" href="#">Action</a></li>
<li><a class="dropdown-item" href="#">Another action</a></li>
<li><a class="dropdown-item" href="#">Something else here</a></li>
</ul>
</li>
</ul>
</div>
</div>
<ul class="navbar-nav justify-content-end">
<li class="nav-item">
<a class="nav-link" href="#">Зарегистрироваться</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Войти</a>
</li>
</ul>
</nav>
{% endblock %}
"cardslider"
{% extends "base.html" %}
{% block cardslider %}
<div class="slider middle">
<div class="slides">
<input type="radio" name="r" id="r1" checked>
<input type="radio" name="r" id="r2">
<input type="radio" name="r" id="r3">
<input type="radio" name="r" id="r4">
<input type="radio" name="r" id="r5">
<div class="slide s1"><img src="" alt=""></div>
<div class="slide"><img src="" alt=""></div>
<div class="slide"><img src="" alt=""></div>
<div class="slide"><img src="" alt=""></div>
<div class="slide"><img src="" alt=""></div>
</div>
<div class="navigation">
<label for="r1" class="bar"></label>
<label for="r2" class="bar"></label>
<label for="r3" class="bar"></label>
<label for="r4" class="bar"></label>
<label for="r5" class="bar"></label>
</div>
</div>
{% endblock %}
Простой
Средний