Looks like product is adding in cart and checkout also work perfectly , i have tried the exact step with the dummy order on the link which you have given above and order gets placed successfully , here is the order id for you reference - 000000046.
i know that order is placing but after clicking on proceed to checkout it doesn't go to payment gateway to pay for items that we have added to our cart and shows no such entity with cart id error.
Looks like you have issue with your PayU money payment integration packet.
As its redirecting on payu screen but showing payment failure error.
you can raise a ticket on payu , they might help you in this case.
Hope it helps
I have check your site and I don't face any issue related to cart id is not found. Might be those error are display at certain time when you stand at payment step and you just reload your browser page at payment step so some time for guest users data will be wiped out from shipping step and when you clicked on placed order button at that time they will display error like you faced.
You can also refer this blog for sort out your issue,
https://codeblog.experius.nl/magento-2-checkout-debuggen/