Shuelloapho57
In Switzerland, seat reservations are (very) optional, meaning that almost nobody does it. See details in this => blog article
Furthermore, seat reservations are only possible on IC (Intercity) and EC (Eurocity) trains + a few “panoramic” dedicated tourist trains.
The suggested connections for your itinerary however do consist of an IR 90 (Interregio) train to Sion and a postal bus afterwards. Therefore seat reservations are not possible on both, which is why no seat reservation option is appearing as additional ticket option in the online timetable / ticket shop.