The national news broadcast on France 2 had a segment last night on the bugs in the new app, making parts of it unreliable and unusable. The developers are working on it, a few bugs at a time. So, it's not you, it's their incompetence.
Bets, I also saw that news report last night (I subscribe to TV5Monde in the US and they carry France 2 news) and I thought of all the people on this forum who have been complaining about the app.
Thankfully, in my experience SNCF customer service is surpringly fast at answering letters sent by post to their customer service! Typically takes just a few days.
And even relatively minor complaints stand a good chance of them giving you a voucher in return.
But yes, the new app is quite bad - previous bookings and options just weren't migrated to it, which is mind-boggling!
Despite the problems, I just found that our senior card numbers and info have transferred over. I also see that luggage delivery is being offered again though I didn't click through to see if it really is live or not.
I looked up prices here on SNCF and then compared with Trainline. https://www.thetrainline.com/en-us
Trainline was half the price for the same train. Am I missing something?