The best way to find the issue causing the aircraft not to arm is to check the status page. If there is a cross, you will need to fix it, for example, "compass not calibrated", you will need to calibrate the compass before INAV will allow arming. If the only cross is navigation is unsafe, then you need to check the GPS signal. You will need a minimum of 6 satellites if you have any GPS modes such as RTH enabled before you can arm. The GPS will not pick up enough satellites indoors or in crowded areas like a city centre. You should allow 15 minutes for first boot or after moving to a significantly different area for the GPS to update and acquire satellites. For all other times, allow 1-2 minutes.