Can't activate indoor position hold using snapdragon flight optic flow

Hi, guys

I’m integrating snapdragon flight board to quad rotor. I already well tune and fly with stabilize mode. Next, I would like to use onboard optic flow to do indoor position hold. When I switched to position mode It was reject to altitude hold. Anyone know this issue and what parameter need to be change. I am not intend to use this outdoor, indoor only.