score:4
Flight prices often go down and up within short periods of time - I have seen specials offered for less than an hour. It could well be that the Google Flights search result is updated only once per day (or every 8 or every 4 hours), so the amount shown is simply outdated - it was available a while ago, but you 'missed it'.
Unfortunately, there are also many providers that trick intentionally with advertising lower prices, and when you try to book, they are 'no longer available' - the idea is that once you progressed that far and already typed in all your personal data, you buy from them anyway. This has become widely normal for hotels, and unfortunately makes the searching for hotels in google maps pretty much unusable, as the shown prices are completely made up and never really available.
Here an example I just pulled - the offers marked up all end with 'no longer available':
Upvote:2
I've seen a similar issue specifically when I search Multi-city instead of a one way. I wanted to book through Chase Ultimate Rewards and got an error, when I called the support desk they said "it must be the airline hasn't updated their inventory", but I kept seeing the same fare multiple days after. In my case, the fare may have existed at some point, but there was no space left under that Fare Class (I searched with ExpertFlyer). It may as well be that the Fare Construction needs to be specified by a travel agent, and that's why Google recommends that but unfortunately I have not found an agent that would do this.