Django Conversion from DecimalField to Decimal is not supported

I was trying to do a cart module from youtube and I ended up encountering thise "TypeError at /store/add-to-cart/1' . And so far I am kind of stuck from this error.

So, here is the code that might (?) be related to this error. my views.py :

def add_to_cart(request, product_id):
product = Product.get_object_or_404(pk= product_id)
order_item, status = OrderItem.objects.get_or_create(product=product)

user_order, status = Order.objects.get_or_create(is_ordered=False)
user_order.items.add(order_item)

if status:
    user_order.OR = generate_order_id()
    user_order.save()

messages.Information(request, 'Item added to cart')
return redirect(reverse('store:index'))

models.py :

class Product(models.Model):
sku = models.CharField(unique = True, max_length = 200)
barcode = models.CharField(max_length = 200)
name = models.CharField(max_length = 50)
description = models.TextField(max_length = 500, blank=True, validators=[MaxLengthValidator(500)])
base_price = models.DecimalField(max_digits = 5, decimal_places = 2, default = 0)
product_image = models.ImageField(blank = True, null = True, upload_to = 'product_images/%Y/%m/%D/')
number_of_stocks = models.IntegerField(default = 0)
date_created = models.DateField(auto_now_add = True)
category = models.ManyToManyField(Category, default = None)

def __str__(self):
    return self.name

# Displays the image
def image_tag(self):
    return u'<img src="%s" width="150" height="150" />' % (self.product_image.upload_to)

image_tag.short_description = 'Image'

class OrderItem(models.Model):
    product = models.OneToOneField(Product, on_delete=models.SET_NULL, null = True)
    is_ordered = models.BooleanField(default=False)
    date_added = models.DateTimeField(auto_now=True)
    date_ordered = models.DateTimeField(null=True)

def __str__(self):
    return self.product.name

class Order(models.Model):
    OR = models.CharField(max_length = 15)
    # user fk
    is_ordered = models.BooleanField(default=False)
    items = models.ManyToManyField(Product, default = None)
    order_date = models.DateTimeField()
    raw_total_price = models.DecimalField(max_digits = 6, decimal_places = 2, default = 0)
    delivery_fee = models.DecimalField(max_digits = 6, decimal_places = 2, default = 150)
    total_price = models.DecimalField(max_digits = 6, decimal_places = 2, default = delivery_fee)
    # order status fk

def get_cart_items(self):
    return self.products.all()

def get_cart_total(self):
    return sum([item.product.base_price for item in self.items.all()])

def __str__(self):
    return self.OR