Beihai to Guangzhou

I think the first class seat "sold out" and thus not available. I noticed that for some trips like this one and Guilin trip, the first class seat always "sold out". However, sometimes they will become available again when the departure date is approaching like a few days in advance. No idea about the reason. You may choose to wait until the date is approaching or book a second class seat first and then change it to first class if thet become available later.