SNCF is quite easy to use - app or website on the laptop.
I am currently monitoring a weekend trip labeled "not bookable". I know there are on-going track works. They won't release the actual availability until the route or a substitute is known. That may be what you are encountering for your travel dates. Rail Europe and other agencies frequently sell tickets before they are confirmed and that later have to be changed to the actual route that the railway company makes available.
You can check an upcoming week to see what might be later released officially by SNCF. At least you can get some notion of what might be available for your dates. Checking two weeks in advance on a weekend a couple months before my actual travel dates, SNCF did show a substitute route that would be acceptable. In other words, I can get from point A to B - not the routing I'd like if track works weren't in process, but SNCF is making the trip possible, posting as info is known.