the reason for such a message is because for the section of your travel in Italy, the Italian operator Trenitalia is using a closed system with mandatory seat reservation so they can charge you extra. And as your train to visp is the one extended to Frankfurt, chance of sold out seat is pretty high. The German and SBB ticket system is an open system, so the ticket cannot be sold out.
the reason again why I said you should book a regionale via the website of Trenitalia from Milano centrale to Domodossola is exactly because of this. So again, the regionale train from Milano centrale to Domodossola is at a regional fare system and it cannot be sold out. From Domodossola on, even it is still in Italy but it is in the Swiss ticket scheme. Which means it is impossible for the point to point ticket to be sold out. As I can show you from what I just checked. But bear in mind if you buy super saver ticket from SBB, even though it is cheaper. You will have to use the specific trains you booked. Not worth the risk. And if you see from my screenshot, the first EC is the one to Frankfurt, which is the one you had problem with most likely, due to the closed system and mandatory seat reservation scam of Trenitalia.
So, the best solution for you is to buy a point to point ticket from Domodossola to visp, and the price is shown in my screenshot (be aware if you don’t have a half fare card the price is doubled) and also book another ticket from Milano centrale to Domodossola via Trenitalia, as SBB cannot sell you an Italian regionale ticket.
I also included the booking page from the app of Trenitalia. And yes, the seat for the eurocity you desired which is exactly the same train you see from the screenshot of SBB app was sold out. Just book the previous regionale and you will be fine. I am truly sorry for you to go through all this confusion and mess.
so here will what you need to do in order to arrive in visp like what you planned. You need to book the Trenord regionale 2418 via Trenitalia, which you can get from the ticket machine or their website/app. And you need to buy the point to point ticket via SBB from Domodossola to visp, it will be valid for the whole day so even in case of delay in Italy you will be able to catch any other train from domo to visp.