It already happened to me twice, when I tried to book a flight on Bravofly/Lastminute, that i couldnt complete the booking, because some error occured after clicking on "book".
So I opened the network tab in dev tools to check what exactly went wrong and found this error message.
Lastminute prevents bookings that they would lose money with. Threshold seems to be at 15€ (per person?).
Thus a lot of prices they are advertising are not even bookable.
In my case I added some stupid insurance for a couple of € and the booking was successful.