King Kong 210GT comes with either the F3 or CC3D (flight controller). Of those, LibrePilot only supports the CC3D.
After setup, the default for arming (Configuration -> Input -> ArmingSettings) is "Always Disarmed" so that you must decide how to arm it and configure it that way. Most people use "Yaw Right" which matches some other flight controllers.
If that is not your problem, then look at System Health (ATTI, STAB etc.) on the Flight Data page to see what your problem may be. Perhaps post a screen capture of that page.