There is no direct trains or flights between the two cities. However, you could take a flight from Chengdu to Xiamen first as several direct flights are available daily on the route. After reaching Ximen, you can take a train or coach to Longyan directly, which costs about 5 hrs. For the flight and train schedules, you can check them on the following pages by yourself: https://www.travelchinaguide.com/china-flights/; https://www.travelchinaguide.com/china-trains/