Why INAV is not arming?

One of the most often asked questions about INAV, or rather most often encountered problems is: "INAV is not arming. Why?" There are at least a few possible reasons for it, but luckily INAV Configurator is smart enough to report them in a nice way. And there can be many reasons: accelerometer calibration, faulty hardware, enabled GPS modes or just no GPS fix. Today I will show you how to identify the real reasons for INAV UAV not arming and give most basic suggestions how to solve those problems.