[Python] Can't visualize the validation error on flask web server

I am trying to show a validation error to the users when they try to sign up with an email or username that's already been taken.

However, despite the fact it works, but I can't see anything on the webpage when I run the web development server.

Here is the Python code for the validation error:

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, SubmitField, BooleanField
from wtforms.validators import DataRequired, Length, Email, EqualTo, ValidationError
from flaskblog.models import User


class RegistrationForm(FlaskForm):
username = StringField('Username',
validators=[DataRequired(), Length(min=2, max=20)])
email = StringField('Email',
validators=[DataRequired(), Email()])
password = PasswordField('Password', validators=[DataRequired()])
confirm_password = PasswordField('Confirm Password',
validators=[DataRequired(), EqualTo('password')])
submit = SubmitField('Sign Up')

def validate_username(self, username):
user = User.query.filter_by(username=username.data).first()
if user:
raise ValidationError('That username is taken. Please choose a different one.')

def validate_email(self, email):
email = User.query.filter_by(email=email.data).first()
if email:
raise ValidationError('That email is taken. Please choose a different one.')


class LoginForm(FlaskForm):
email = StringField('Email',
validators=[DataRequired(), Email()])
password = PasswordField('Password', validators=[DataRequired()])
remember = BooleanField('Remember Me')
submit = SubmitField('Login')


But even if the outline is red, I don't see the message:



I'm not sure if it's useful, but here's also the layout.html code I use for generic rendering of pages:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

<link rel="stylesheet" type="text/css" href="{{ url_for('static', filename='main.css') }}">

{% if title %}
<title>Flask Blog - {{ title }} </title>
{% else %}
<title>Flask Blog</title>
{% endif %}
</head>
<body>
<header class="site-header">
<nav class="navbar navbar-expand-md navbar-dark bg-steel fixed-top">
<div class="container">
<a class="navbar-brand mr-4" href="/">Flask Blog</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarToggle" aria-controls="navbarToggle" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarToggle">
<div class="navbar-nav mr-auto">
<a class="nav-item nav-link" href="{{ url_for('home') }}">Home</a>
<a class="nav-item nav-link" href="{{ url_for('about') }}">About</a>
</div>
<!-- Navbar Right Side -->
<div class="navbar-nav">
<a class="nav-item nav-link" href="{{ url_for('login') }}">Login</a>
<a class="nav-item nav-link" href="{{ url_for('register') }}">Register</a>
</div>
</div>
</div>
</nav>
</header>
<main role="main" class="container">
<div class="row">
<div class="col-md-8">
{% with messages = get_flashed_messages(with_categories=true) %}
{% if messages %}
{% for category, message in messages %}
<div class="alert alert-{{ category }}">
{{ message }}
</div>
{% endfor %}
{% endif %}
{% endwith %}
{% block content %}{% endblock %}

</div>
<div class="col-md-4">
<div class="content-section">
<h3>Our Sidebar</h3>
<p class='text-muted'>You can put any information here you'd like.
<ul class="list-group">
<li class="list-group-item list-group-item-light">Latest Posts</li>
<li class="list-group-item list-group-item-light">Announcements</li>
<li class="list-group-item list-group-item-light">Calendars</li>
</ul>
</p>
</div>
</div>
</div>
</main>


<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>

</body>
</html>

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