Any mode that needs GPS has additional minimum requirements (there are others that are good but not required):
- properly mounted and configured GPS
- twisted high current cables (ALL of them from battery to motor)
- home location set
- working, configured and calibrated external/aux mag
- INS13 (outdoor/navigation) selected for Attitude Estimation Algorithm
- properly set failsafes (RC and FC) (this is required even for non-GPS flight)
Some of these missing (the ones that can be detected) will keep it from arming, some missing will make it fly away or crash.