在TGV网站上找到的低价车票，怎么写着没有座位

我在法国高铁网站找到的低价车票，从巴黎到图尔的，显示没有座位，请问是怎么回事啊，在线等。。。。非常感谢。显示 On some trains, the place you requested isn't available for the fare you chose. We have reserved you a place with different criteria instead.
Attention, some of your requests for seats cannot be satisfied.