Gimbal ROLL/TILT setup - Not working for ArduPlane

I am trying to set up camera ROLL Gimbal control on AP 3.6.0, X2 board and latest mission planner 1.39.1.x. it seems impossible to update the Camera Gimbal ROLL or TILT values (see attached image) - Mission planner reports error and will not write PWM & GAIN to eeprom.

Why is that? Can’t i use this Tab for Arduplane but only for ArduCopter?