I have received error 301 - Option Not Found In Valuation in my response. What does it mean?

Error code 301 will always appear at Valuation/Quote step as it means that the selected option by the Buyer in the previous step (Availability/Search) isn't available anymore. 

But, why does exactly does this error appear and what is the main difference with the error code 204 - No results found?  

One of our commitments is matching our Booking flow with the Seller own booking flow and in most cases, our Valuation method has to run more than one procedure. This means that it's very common that our Valuation is composed by a second search to the Seller's system in order to figure out if the selected option is still available. In case that we return an error 204, it means that the Seller returned 0 options and otherwise error 301 if the availability selected option is not returned. 

In order to minimize this type of error try to reduce the time between the search performed and the pre-book request. If the error persists, please contact our Help Center.