the validators message doesn't show

from flask import (Flask, g, render_template, flash, redirect, url_for)
from flask_login import LoginManager

import forms
import models

DEBUG = True
PORT = 8000
HOST = '127.0.0.1'

app = Flask(__name__)
app.secret_key = "dfskjdfhjk3.4340sokfldskl;sker;wk;ritipe2934u3o5990jskjfkwej!"

login_manager = LoginManager()
login_manager.init_app(app)
login_manager.login_view = 'login'

@login_manager.user_loader
def load_user(userid):
    try:
        return models.User.get(models.User.id == userid)
    except models.DoesNotExist:
        return None


@app.before_request
def before_request():
    """Connect to the database before each request."""
    g.db = models.DATABASE
    g.db.connect()


@app.after_request
def after_request(response):
    """Close the database connection after each request."""
    g.db.close()
    return response


@app.route('/register', methods=('GET', 'POST'))
def register():
    form = forms.RegisterForm()
    if form.validate_on_submit():
        flash('Yay, you registered!', 'success')
        models.User.create_user(
            username=form.username.data,
            email=form.email.data,
            password=form.password.data
        )
        return redirect(url_for('index'))
    return render_template('register.html', form=form)


@app.route('/')
def index():
    return 'Hey'


if __name__ == '__main__':
    models.initialize()
    try:
        models.User.create_user(
            username="kennethlove",
            email="kenneth@teamtreehouse.com",
            password="password",
            admin=True
        )
    except ValueError:
        pass
    app.run(debug=DEBUG, host=HOST, port=PORT)

import datetime

from flask_bcrypt import generate_password_hash
from flask_login import UserMixin
from peewee import *


DATABASE = SqliteDatabase("social.db")


class User(UserMixin, Model):
    username = CharField(unique=True)
    email = CharField(unique=True)
    password = CharField(max_length=100)
    joined_at = DateTimeField(default=datetime.datetime.now)
    is_admin = BooleanField(default=False)

    class Meta:
        database = DATABASE
        order_by = ('-joined_at',)

    @classmethod
    def create_user(cls, username, email, password, admin=False):
        try:
            cls.create(
                username=username,
                email=email,
                password=generate_password_hash(password),
                is_admin=admin)
        except IntegrityError:
            raise ValueError('User already exists')


def initialize():
    DATABASE.connect()
    DATABASE.create_tables([User], safe=True)
    DATABASE.close()

from flask_wtf import Form
from wtforms import StringField, PasswordField
from wtforms.validators import (DataRequired, Regexp, ValidationError,
                                Length, EqualTo, Email)

from models import User


def name_exists(form, field):
    if User.select().where(User.username == field.data).exists():
        raise ValidationError("User with that name already exists.")


def email_exists(form, field):
    if User.select().where(User.email == field.data).exists():
        raise ValidationError("User with that email already exists.")


class RegisterForm(Form):
    username = StringField(
        'Username',
        validators=[
            DataRequired(),
            Regexp(
                r'^[a-zA-Z0-9_]+$',
                message=("Username should be one word, letters, "
                         "numbers, and underscores only.")
            ),
            name_exists
        ])

    email = StringField(
        'Email',
        validators=[
            DataRequired(),
            Email(),
            email_exists
        ])
    password = PasswordField(
        'Password',
        validators=[
            DataRequired(),
            Length(min=2),
            EqualTo('password2', message="Passwords must match")
        ])
    password2 = PasswordField(
        'Confirm Password',
        validators=[DataRequired()]
    )

{% from 'macros.html' import render_field %}

<form method="POST" action="" class="form">
    {{ form.hidden_tag() }}
    {% for field in form %}
        {{ render_field(field) }}
    {% endfor %}
    <button type="submit" id="submit">Register</button>
</form>

{% macro render_field(field) %}
    <div class="field">
        {% if field.errors %}
            {% for error in field.errors %}
                <div class="notification error">{{ error }}</div>
            {% endfor %}
        {% endif %}
        {{ field(placeholder=field.label.text) }}
    </div>
{% endmacro %}