Read this page:
https://www.seat61.com/France-trains...20for%20France
TGVs require reservations so theoretically they could sell out, but it's unlikely.
When you buy tickets online it should tell you whether the ticket can be changed, sometimes for a fee.
If you mean the 2nd of May, then there seems to be a Thalys train at 1214 and then a TGV train at 1307, the Thalys train is a lot faster, but the ticket can't be changed after departure, which doesn't help if you already missed the train.
The TGV has an option for a flexible ticket which says it can be changed without fee up to 2 hours after departure.
There are also additional complications if you are using a US bank card or buying on a device which appears to be located in the US (or outside of Europe at least), but the above site goes into this in detail.
TGV ticket machines basically connect to the SNCF website and use the same booking interface, and only take card.