ORM does not work this way.
Item.product is of type Product and item in OrderItem is of type Item
If you want to get Item from slug field on Product, you need to access related object reference using __ (double underscore) in filter condition.
item = Items.objects.filter(product__slug=slug).first()
order_item, created = OrderItem.objects.get_or_create(item=item, account=request.user, ordered=False)
.first() is used because, filter returns QuerySet