Remote command “DO_SET_MODE:DENIED” shown in terminal.cannot find out what limited it.

Can have multiple reasons. For example if you try to change to a mode that requires GPS lock, and you no not have it, it will deny the mode change

yeah,I agree with you,and I plan to make copter takeoff without GPS automatically.If there is a mode support it?
In addition,command of mavlink:DO_SET_MODE ‘s first field base_mode only support MAV_MODE_PREFLIGHT
MAV_MODE_STABILIZE_DISARMED…all modes I tested were denied.