I will buy a ready made drone and would like to fit an autopilot software for predefined path without GPS as its a indoor facility. Looking a solution for autonomous flight software that enables my drone to fly in a warehouse with preset flight path. If no ready made software available, how can I build it for my drone
You will need some sort of localization.
This could be a vision-based system, motion-capture tools or something of that nature. Without position control, you cannot achieve stable autonomous flight.