Can't get error message to show with WTForms _formhelpers to show on Flask webpage

Apologies if this isn't worded very well. I can't seem to get any error messages to show when I haven't fulfilled the validation requirements. When I submit, the form is just refreshed without error messages such as "This field is required".

Also, when I do input data, if form.validate_on_submit(): doesn't return the expected homepage, it once again just refreshes the page. I'm aware the form does nothing, I plan on adding the data submitted to a database but I'm stuck on this step. Can anybody help? Thanks.

app.py

from cs50 import SQL
from flask import Flask, render_template, session, redirect
from flask_session import Session
from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, BooleanField #bool for checkbox
from wtforms.validators import InputRequired, Email, Length


# configure application
app = Flask(__name__)
app.config["SESSION_PERMENANT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)

# Allows webpage to update html on refresh
app.config["TEMPLATES_AUTO_RELOAD"] = True
app.config['SECRET_KEY'] = 'Itsasecret'
# configure CS50 library to use SQLite database
db = SQL("sqlite:///project.db")

# Usually separate routes, database and form stuff into different files and link together

class LoginForm(FlaskForm):
    username =  StringField('Username', validators=[InputRequired(), Length(min=4, max=15)])
    password = PasswordField('Password', validators=[InputRequired(), Length(min=8, max=80)]) #80 is a special number (learn more)
    remember = BooleanField('Remember me')

@app.route('/')
def homepage():
    return render_template("homepage.html")

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        return redirect("/")
    return render_template("login.html", form=form)

login template

{% extends "layout.html" %}
{% from "_formhelpers.html" import render_field %}

{% set active_page = 'login' %}

{% block title %} Login {% endblock %}

{% block body %}
<div class="ls-main">
    <form action="/login" method="POST">
        {{ form.csrf_token }}        
        {{ render_field(form.username) }}
        {{ render_field(form.password, autocomplete="off") }}
        {{ render_field(form.remember) }}
        <div class="ls-div">
            <input class="ls-input, signinbutton" name="submit" type="submit" value="Submit">
        </div>

    </form>
</div>
{% endblock %}

_formhelpers.html

{% macro render_field(field) %}
  <dt>{{ field.label }}
  <dd>{{ field(**kwargs)|safe }}
  {% if field.errors %}
    <ul class=errors>
    {% for error in field.errors %}
      <li>{{ error }}</li>
    {% endfor %}
    </ul>
  {% endif %}
  </dd>
{% endmacro %}

return render_template("/") is invalid. I think you mean return redirect("/")

Of course you need from flask import Flask, render_template, session, redirect