Avanti continue to falsely mark their trains as "reservations compulsory" in the timetable data, meaning that you cannot buy a ticket in conjunction with an itinerary involving one of their services unless they have released reservations. They haven't done so for dates that far ahead yet - so you'll have to wait if you want a reservation.
You can already buy walk-up tickets now without a reservation for an Avanti service, but given that these will obviously cost the same even if bought on the day, there's little point in doing so. Best to
sign up for alerts for when Avanti release Advance tickets. They're currently selling weekend Advances until 2 July, so you likely have at least a month or so to wait.