Django This field is required

from django import forms
    
PRODUCT_QUANTITY_CHOICES = [(i, str(i)) for i in range(1, 21)]
 
 
    class CartAddProductForm(forms.Form):
        quantity = forms.TypedChoiceField(choices=PRODUCT_QUANTITY_CHOICES, coerce=int)
        update = forms.BooleanField(required=False, initial=False, widget=forms.HiddenInput)

from django.shortcuts import render, redirect, get_object_or_404
from django.views.decorators.http import require_POST
from shop.models import Product
from .cart import Cart
from .forms import CartAddProductForm
 
 
@require_POST
def cart_add(request, product_id):
    cart = Cart(request)
    product = get_object_or_404(Product, id=product_id)
    form = CartAddProductForm(request.POST)
 
    if form.is_valid():
        cd = form.cleaned_data
        cart.add(product=product,
                 quantity=cd['quantity'],
                 update_quantity=cd['update'])
    print(form.errors)
    return redirect('cart:cart_detail')
 
 
def cart_remove(request, product_id):
    cart = Cart(request)
    product = get_object_or_404(Product, id=product_id)
    cart.remove(product)
    return redirect('cart:cart_detail')
 
 
def cart_detail(request):
    cart = Cart(request)
    return render(request, 'cart/detail.html', {'cart': cart})

from decimal import Decimal
from django.conf import settings
from shop.models import Product
 
 
class Cart(object):
 
    def __init__(self, request):
 
        self.session = request.session
        cart = self.session.get(settings.CART_SESSION_ID)
        if not cart:
            # save an empty cart in the session
            cart = self.session[settings.CART_SESSION_ID] = {}
        self.cart = cart
 
 
    def save(self):
       
        self.session[settings.CART_SESSION_ID] = self.cart
        
        self.session.modified = True
 
    def add(self, product, quantity=1, update_quantity=False):
      
        product_id = str(product.id)
        if product_id not in self.cart:
            self.cart[product_id] = {'quantity': 0,
                                     'price': str(product.price)}
        if update_quantity:
            self.cart[product_id]['quantity'] = quantity
        else:
            self.cart[product_id]['quantity'] += quantity
        self.save()
 
 
    def remove(self, product):
     
        product_id = str(product.id)
        if product_id in self.cart:
            del self.cart[product_id]
            self.save()
 
    def __iter__(self):
 
        product_ids = self.cart.keys()
       
        products = Product.objects.filter(id__in=product_ids)
        for product in products:
            self.cart[str(product.id)]['product'] = product
 
        for item in self.cart.values():
            item['price'] = Decimal(item['price'])
            item['total_price'] = item['price'] * item['quantity']
            yield item
 
    def __len__(self):
 
        return sum(item['quantity'] for item in self.cart.values())
 
    def get_total_price(self):
 
        return sum(Decimal(item['price']) * item['quantity'] for item in
                   self.cart.values())
 
    def clear(self):
        del self.session[settings.CART_SESSION_ID]
        self.session.modified = True

 
    def add(self, product, quantity=1, update_quantity=False):
      
        product_id = str(product.id)
        if product_id not in self.cart:
            self.cart[product_id] = {'quantity': 0,
                                     'price': str(product.price)}