How could I go from Fenghuang to Chengdu in the same day?

I’m afraid the time is not enough for you to catch the train at 08:36. As I know, the earliest bus to Huaihua usually departs at 07:00, and it takes 1.5-2 hours to get there.
You are advised to take the bus to Tongren at first. This bus usually leaves before 07:00 in the morning. It will take around 1 hour to Tongren. Then you may take a taxi to the railway station to catch the train which departs at 09:44.

Thank you so much Jenny.
I'm going to go to China in June. I think this route, maybe I would do it on a Monday, to avoid the weekends
Do you think I should buy the train tickets in advance, or in June could I buy the train tickets directly at the train stations?
Thanks

You can buy the ticket at the train stations directly. Generally speaking, it will not be crowed at that time, especially you will do it on weekdays. It could be a long trip from Tongren to Chengdu, so you are advised to buy a berth ticket. You may pay more attention to the remaining tickets of your departure day.