Online train reservation: Nagano to Matsumoto

Trying to make a train reservation ahead of arriving in Japan.

Nagano to Matsumoto on the Wide View Shinano.

Can it be true that none of the online reservation systems of JR Central, JR West and JR East offer the option to make a reservation for this?

Yes, it is possible on the JR East Reservation website:
https://www.eki-net.com/en/jreast-train-reservation/reserve/wb/RouteSe...