Without meaning to sound rude, what happens when you try and book on the QR website directly?
Start it as a new booking, without being linked by Google Flights, and follow it through.
Google Flights could well be showing phantom availability, and it has history of doing so. The price you are seeing on Google Flights, may simply no longer be available, hence it is triggering an error.
Try a brand new booking on the QR website and let us know if that solves the issue?
Best,
M