Can't change to offboard with rc

i try to change it to the Offboard mode, pixhawk’s red light blinks for maybe 4 times, with buzzer rings 3 times, and can’t change it to offboard.
i think i need to send several position commands to it before, but everything seems the same.

and someone said that after the drone fly in the sky, change to alt-hold(may be other name i guess), then change to offboard and success, but i manage to change to position mode , but fail to change to offboard with the same situation like this. neither with rc nor mavros. do i miss some steps or so?
right now i arm it, speed up the throttle(not with porps), change to altitude mode, and then try to change it to offboard, but failed.

now i am able to make it to the Offboard mode by publish /mavros/setpoint plugins!