This route was just approved by the US DOT as a fifth freedom route which allows airlines to establish connecting routes in foreign countries as long as it is a continuation of service beginning in the airline's home country. This route technically begins in Beijing, but for US-based passengers, they are allowed to buy the last leg of the route from Houston to Panama City.
This route is so new that Air China doesn't even have Panama City listed on its website for booking. For the time being, you will have to book this flight via a third-party site.
Searching on Priceline we found flights departing IAH on Thursday, April 12 and returning from PTY on Sunday, April 15, 2018. This flight will only operate on Sundays and Thursdays.
Need a room while you are in Panama City? Here is what is available for the listed dates on Hotelwatchdog.