Fukuoka airport to Kumamoto via SunQ pass

I will be arriving at Fukuoka airport at around 7pm and would take a bus going to Kumamoto Kotsu via the Sunq pass. Is there a direct bus from the airport? Do I need to make a reservation? I tried to inquire via https://global.atbus-de.com/route_lists/?locale=en but it says "No vacant seat to match with the conditions. Please try to change the conditions. " I'm also wondering what the "(seating capacity)" means here?

Now checked and understood.

1)
Yes, operating everyday also after 19:00 still operated.
http://www.nishitetsu.jp/bus/highwaybus/rosen/hinokuni.html
http://www.nnr.co.jp/bus/highway/highwaybus_kumamoto.pdf
Fukuoka airport(int'l/dom) : 福岡空港(国際線/国内線)
Kumamoto kotsu center(bus terminal/near castle) : 熊本交通センター
平日：weekday(MON-FRI)
土休日(in RED) ： SAT,SUN and National holidays(23 Dec, 1 Jan)

But in yr case take dep fr Hakata bus terminal/博多バスターミナル one fastest.
(take subway first/SunQ pass not covered)
BC direct bus fr FUK is "all stop(各停)" type slower by no skipping.

2)
No.
This route bus(Hinokuni-gou/ひのくに号) is NO seat reservation(予約不要) type.
Dep fr Hakata bus terminal ones,too. (except last 2 buses in late night)

Just listed as searching selection menu.
In JP reservation form no listed so in this case one of small bug.
https://goo.gl/gYVKWp

3)
"seating capacity" means No accept standing passengers. (座席定員制)
So seat numbers(about 50) is passenger numbers limit(=capacity).
http://www.nishitetsu.jp/bus/highwaybus/rosen/hinokuni.html

No accept standing passengers reason is very simple.
BC this bus route on ExpWay(high speed way/toll section) mainly
passengers have to wear "seat-belt/シ−トベルト" by Bus traffic Law.
http://www.miebus-kyokai.or.jp/information/images/info200806.gif
http://www.jnto.go.jp/eng/arrange/transportation/auto/bus.html